Apparatuses and methods of ocular surface interferometry (OSI) employing polarization and subtraction for imaging, processing, and/or displaying an ocular tear film

ABSTRACT

Apparatuses and methods employing ocular surface interferometry (OSI) employing polarization and subtraction for imaging, processing, and/or displaying an ocular tear film are disclosed. The apparatuses and methods can be employed for measuring tear film layer thickness (TFLT) of the ocular tear film, which includes lipid layer thickness (LLT) and/or aqueous layer thickness (ALT). An imaging device is focused on the lipid layer of the tear film to capture optical wave interference interactions of specularly reflected light from the tear film combined with a background signal(s) in a first image. The imaging device is focused on the lipid layer of the tear film to capture a second image containing background signal(s) in the first image. The second image can be subtracted from the first image to reduce and/or eliminate background signal(s) in the first image to produce a resulting image that can be analyzed to measure tear film layer thickness (TFLT).

PRIORITY APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication No. 61/638,231 entitled “APPARATUSES AND METHODS OF OCULARSURFACE INTERFEROMETRY (OSI) EMPLOYING POLARIZATION AND SUBTRACTION FORIMAGING, PROCESSING, AND/OR DISPLAYING AN OCULAR TEAR FILM,” filed Apr.25, 2012, which is incorporated herein by reference in its entirety.

The present application is also a continuation-in-part application ofU.S. patent application Ser. No. 12/798,325 entitled “OCULAR SURFACEINTERFEROMETRY (OSI) METHODS FOR IMAGING, PROCESSING, AND/OR DISPLAYINGAN OCULAR TEAR FILM,” filed Apr. 1, 2010, which claims priority to U.S.Provisional Patent Application No. 61/211,596 entitled “OCULAR SURFACEINTERFEROMETRY (OSI) DEVICES, SYSTEMS, AND METHODS FOR MEASURING TEARFILM LAYER THICKNESS(ES),” filed on Apr. 1, 2009, which are bothincorporated herein by reference in their entireties.

RELATED APPLICATIONS

The present application is related to concurrently filed U.S. patentapplication Ser. No. 13/870,214 entitled “BACKGROUND REDUCTIONAPPARATUSES AND METHODS OF OCULAR SURFACE INTERFEROMETRY (OSI) EMPLOYINGPOLARIZATION FOR IMAGING, PROCESSING, AND/OR DISPLAYING AN OCULAR TEARFILM”, filed on Apr. 25, 2013, which claims priority to U.S. ProvisionalPatent Application No. 61/638,260 entitled “BACKGROUND REDUCTIONAPPARATUSES AND METHODS OF OCULAR SURFACE INTERFEROMETRY (OSI) EMPLOYINGPOLARIZATION FOR IMAGING, PROCESSING, AND/OR DISPLAYING AN OCULAR TEARFILM,” filed on Apr. 25, 2012, both of which are incorporated herein byreference in their entireties.

The present application is also related to U.S. patent application Ser.No. 11/820,664 entitled “TEAR FILM MEASUREMENT,” filed on Jun. 20, 2007,issued as U.S. Pat. No. 7,758,190, which is incorporated herein byreference in its entirety.

The present application is also related to U.S. patent application Ser.No. 11/900,314 entitled “TEAR FILM MEASUREMENT,” filed on Sep. 11, 2007,issued as U.S. Pat. No. 8,192,026, which is incorporated herein byreference in its entirety.

The present application is also related to U.S. patent application Ser.No. 12/798,275 entitled “OCULAR SURFACE INTERFEROMETRY (OSI) DEVICES ANDSYSTEMS FOR IMAGING, PROCESSING, AND/OR DISPLAYING AN OCULAR TEAR FILM,”filed on Apr. 1, 2010, which is incorporated herein by reference in itsentirety.

The present application is also related to U.S. patent application Ser.No. 12/798,326 entitled “OCULAR SURFACE INTERFEROMETRY (OSI) METHODS FORIMAGING AND MEASURING OCULAR TEAR FILM LAYER THICKNESS(ES),” filed onApr. 1, 2010, issued as U.S. Pat. No. 8,092,023, which is incorporatedherein by reference in its entirety.

The present application is also related to U.S. patent application Ser.No. 12/798,324 entitled “OCULAR SURFACE INTERFEROMETRY (OSI) DEVICES ANDSYSTEMS FOR IMAGING AND MEASURING OCULAR TEAR FILM LAYER THICKNESS(ES),”filed on Apr. 1, 2010, issued as U.S. Pat. No. 8,215,774, which isincorporated herein by reference in its entirety.

The present application is being filed with three (3) sets of colorversions of the drawings discussed and referenced in this disclosure.Color drawings more fully disclose the subject matter disclosed herein.

FIELD OF THE DISCLOSURE

The technology of the disclosure relates to imaging a mammalian oculartear film. The technology of the disclosure also relates to measuringocular tear film layer thickness(es), including lipid layer thickness(LLT) and/or aqueous layer thickness (ALT). Imaging the ocular tear filmand measuring TFLT may be used to diagnose “dry eye,” which may be dueto any number of deficiencies, including lipid deficiency and aqueousdeficiency.

BACKGROUND

In the human eye, the precorneal tear film covering ocular surfaces iscomposed of three primary layers: the mucin layer, the aqueous layer,and the lipid layer. Each layer plays a role in the protection andlubrication of the eye and thus affects dryness of the eye or lackthereof. Dryness of the eye is a recognized ocular disease, which isgenerally referred to as “dry eye,” “dry eye syndrome” (DES), or“keratoconjunctivitis sicca” (KCS). Dry eye can cause symptoms, such asitchiness, burning, and irritation, which can result in discomfort.There is a correlation between the ocular tear film layer thicknessesand dry eye disease. The various different medical conditions and damageto the eye as well as the relationship of the aqueous and lipid layersto those conditions are reviewed in Surv Opthalmol 52:369-374, 2007 andadditionally briefly discussed below.

As illustrated in FIG. 1, the precorneal tear film includes an innermostlayer of the tear film in contact with a cornea 10 of an eye 11 known asthe mucus layer 12. The mucus layer 12 is comprised of many mucins. Themucins serve to retain aqueous in the middle layer of the tear filmknown as the aqueous layer. Thus, the mucus layer 12 is important inthat it assists in the retention of aqueous on the cornea 10 to providea protective layer and lubrication, which prevents dryness of the eye11.

A middle or aqueous layer 14 comprises the bulk of the tear film. Theaqueous layer 14 is formed by secretion of aqueous by lacrimal glands 16and accessory tear glands 17 surrounding the eye 11, as illustrated inFIG. 2. The aqueous, secreted by the lacrimal glands 16 and accessorytear glands 17, is also commonly referred to as “tears.” One function ofthe aqueous layer 14 is to help flush out any dust, debris, or foreignobjects that may get into the eye 11. Another important function of theaqueous layer 14 is to provide a protective layer and lubrication to theeye 11 to keep it moist and comfortable. Defects that cause a lack ofsufficient aqueous in the aqueous layer 14, also known as “aqueousdeficiency,” are a common cause of dry eye. Contact lens wear can alsocontribute to dry eye. A contact lens can disrupt the natural tear filmand can reduce corneal sensitivity over time, which can cause areduction in tear production.

The outermost layer of the tear film, known as the “lipid layer” 18 andalso illustrated in FIG. 1, also aids to prevent dryness of the eye. Thelipid layer 18 is comprised of many lipids known as “meibum” or “sebum”that is produced by meibomian glands 20 in upper and lower eyelids 22,24, as illustrated in FIG. 3. This outermost lipid layer is very thin,typically less than 250 nanometers (nm) in thickness. The lipid layer 18provides a protective coating over the aqueous layer 14 to limit therate at which the aqueous layer 14 evaporates. Blinking causes the uppereyelid 22 to mall up aqueous and lipids as a tear film, thus forming aprotective coating over the eye 11. A higher rate of evaporation of theaqueous layer 14 can cause dryness of the eye. Thus, if the lipid layer18 is not sufficient to limit the rate of evaporation of the aqueouslayer 14, dryness of the eye may result.

Notwithstanding the foregoing, it has been a long standing and vexingproblem for clinicians and scientists to quantify the lipid and aqueouslayers and any deficiencies of same to diagnose evaporative tear lossand/or tear deficiency dry eye conditions. Further, many promisingtreatments for dry eye have failed to receive approval from the UnitedStates Food and Drug Administration due to the inability to demonstrateclinical effectiveness to the satisfaction of the agency. Manyclinicians diagnose dry eye based on patient symptoms alone.Questionnaires have been used in this regard. Although it seemsreasonable to diagnose dry eye based on symptoms alone, symptoms ofocular discomfort represent only one aspect of “dry eyes,” as defined bythe National Eye Institute workshop on dry eyes. In the absence of ademonstrable diagnosis of tear deficiency or a possibility of excessivetear evaporation and damage to the exposed surface of the eye, onecannot really satisfy the requirements of dry eye diagnosis.

SUMMARY OF THE DETAILED DESCRIPTION

Embodiments of the detailed description include ocular surfaceinterferometry (OSI) devices, systems, and methods for imaging an oculartear film and/or measuring a tear film layer thickness (TFLT) of amammalian's ocular tear film. The OSI devices, systems, and methods canbe used to measure the thickness of the lipid layer component (LLT)and/or the aqueous layer component (ALT) of the ocular tear film. “TFLT”as used herein includes LLT, ALT, or both LLT and ALT. “Measuring TFLT”as used herein includes measuring LLT, ALT, or both LLT and ALT. Imagingthe ocular tear film and measuring TFLT can be used in the diagnosis ofthe tear film, including but not limited to lipid layer and aqueouslayer deficiencies. These characteristics may be the cause orcontributing factor to a patient experiencing dry eye syndrome (DES).

In this regard, embodiments disclosed herein include a multi-wavelengthlight source that is controlled to direct light in the visible region toan ocular tear film. The light source may be a Lambertian emitter thatprovides a uniform or substantially uniform intensity in all directionsof emission. The light source is arranged such that light rays emittedfrom the light source are specularly reflected from the tear film andundergo constructive and destructive optical wave interferenceinteractions (also referred to as “interference interactions”) in theocular tear film. An imaging device having a detection spectrum thatincludes the spectrum of the light source is focused on an area(s) ofinterest on the lipid layer of the tear film. The imaging devicecaptures the interference interactions (i.e., modulation) of specularlyreflected light rays from the illuminated tear film coming together bythe focusing action of the imaging device in a first image. The imagingdevice then captures the optical wave interference signals (alsoreferred to as “interference signals”) representing the interferenceinteractions of specularly reflected light from the tear film. Theimaging device produces an output signal(s) representative of theinterference signal in a first image. The first image may contain aninterference signal for a given imaged pixel or pixels of the lipidlayer by the imaging device.

The first image can be displayed to a technician or other user. Thefirst image can also be processed and analyzed to measure a TFLT in thearea or region of interest of the ocular tear film. In one embodiment,the first image also contains a background signal(s) that does notrepresent specularly reflected light from the tear film which issuperimposed on the interference signal(s). The first image is processedto subtract or substantially subtract out the background signal(s)superimposed upon the interference signal to reduce error before beinganalyzed to measure TFLT. This is referred to as “backgroundsubtraction” in the present disclosure. The separate backgroundsignal(s) includes returned captured light that is not specularlyreflected from the tear film and thus does not contain optical waveinterference information (also referred to as “interferenceinformation”). For example, the background signal(s) may include stray,ambient light entering into the imaging device, scattered light from thepatient's face and eye structures outside and within the tear film as aresult of ambient light and diffuse illumination by the light source,and eye structure beneath the tear film, and particularly contributionfrom the extended area of the source itself. The background signal(s)adds a bias (i.e., offset) error to the interference signal(s) therebyreducing interference signal strength and contrast. This error canadversely influence measurement of TFLT. Further, if the backgroundsignal(s) has a color hue different from the light of the light source,a color shift can also occur to the captured optical wave interference(also referred to as “interference”) of specularly reflected light thusintroducing further error.

In this regard in one embodiment, an apparatus for imaging an oculartear film is disclosed. The apparatus includes a control systemconfigured to receive at least one first image containing optical waveinterference of specularly reflecting light in a first polarizationplane along with a background signal from a region of interest (ROI) ofthe ocular tear film captured by an imaging device while illuminated bythe multi-wavelength light source. The control system is also configuredto receive at least one second image containing the background signal ina second polarization plane perpendicular or substantially perpendicularto the first polarization plane from the ROI of the ocular tear filmcaptured by an imaging device. In this manner, an imaging devicecaptures background signal(s) in a second image that is representativeof the signal which is superimposed on the interference of thespecularly reflecting light from the tear film in the first image. Thesecond image is subtracted from the first image to produce a resultingimage having isolated interference signal components. The resultingimage can then be displayed on a visual display to be analyzed by atechnician and/or processed and analyzed to measure a TFLT. Onenon-limiting benefit of the apparatus is that it allows capturing the atleast one second image containing the background signal in a secondpolarization plane perpendicular or substantially perpendicular to thefirst polarization plane from the ROI of the ocular tear film. As aresult, the background signal is isolated from the interference of thespecularly reflecting light from the tear film. Thus, a backgroundoffset captured in the at least one first image is removed or reducedfrom at least one resulting image generated by the subtraction of the atleast one second image from the at least one first image.

In another embodiment, a method of imaging an ocular tear film isdisclosed. The disclosed method involves illuminating the ROI of anocular tear film with the multi-wavelength light source. The methodincludes capturing optical wave interference of specularly reflectedlight in a first polarization plane including a background signal fromthe ROI of the ocular tear film while illuminated by themulti-wavelength light source in at least one image by an imagingdevice. The method also includes capturing the background signal in asecond polarizing plane perpendicular or substantially perpendicular tothe first polarization plane from the ROI of the ocular tear film in atleast one second image by an imaging device. The method also includessubtracting the at least one second image from the at least one firstimage to generate at least one resulting image containing the opticalwave interference of specularly reflected light from the ROI of theocular tear film with the background signal removed. Capturing the atleast one second image containing the background signal in a secondpolarization plane perpendicular or substantially perpendicular to thefirst polarization plane from the ROI of the ocular tear film canisolate the background signal from the interference of the specularlyreflecting light from the ocular tear film. In this manner, a backgroundoffset captured in the at least one first image is removed or reducedfrom at least one resulting image generated by the subtraction of the atleast one second image from the at least one first image.

After the interference of the specularly reflected light is captured anda resulting image containing the interference signal is produced fromany method or device disclosed in this disclosure, the resulting imagecan also be pre-processed before being processed and analyzed to measureTFLT. Pre-processing can involve performing a variety of methods toimprove the quality of the resulting signal, including but not limitedto detecting and removing eye blinks or other signals in the capturedimages that hinder or are not related to the tear film. Afterpre-processing, the interference signal or representations thereof canbe processed to be compared against a tear film layer interference modelto measure TFLT. The interference signal can be processed and convertedby the imaging device into digital red-green-blue (RGB) component valueswhich can be compared to RGB component values in a tear filminterference model to measure TFLT on an image pixel-by-pixel basis. Thetear film interference model is based on modeling the lipid layer of thetear film in various thicknesses and mathematically or empiricallyobserving and recording resulting interference interactions ofspecularly reflected light from the tear film model when illuminated bythe light source and detected by a camera (imaging device).

In a tear film interference model, the lipid layer is modeled of variousLLTs to observe interference interactions resulting from the variousLLTs. The aqueous layer may be modeled in the tear film interferencemodel to be of an infinite, minimum, or varying thickness. If theaqueous layer is modeled to be of an infinite thickness, the tear filminterference model assumes no specular reflections occur from theaqueous-to-mucin layer transition. If the aqueous layer is modeled to beof a certain minimum thickness (˜>2 μm e.g.), the effect of specularreflection from the aqueous-to-mucin layer transition may be consideredin the resulting interference. In either case, the tear filminterference model is a 2-wave tear film interference model to representthe interference between specularly reflected light from the air-tolipid layer transition and the lipid-to-aqueous layer transition. Thus,a 2-wave tear film interference model will include one-dimension of datacomprised of interference interactions corresponding to the variousLLTs. In this case, to measure LLT the interference interactions in theinterference signal representing specularly reflected light from thetear film produced by the imaging device are compared to theinterference patterns in the tear film interference model. However, ifthe aqueous layer is also modeled to be of varying ALTs, the tear filminterference model will be a 3-wave tear film interference model. The3-wave tear film interference model will include interference betweenthe air-to lipid layer, lipid-to-aqueous layer, andaqueous-to-mucus/cornea layer transitions. As a result, a 3-wave tearfilm interference model will include two-dimensions of data comprised ofinterference interactions corresponding to various LLT and ALTcombinations. In this case, to measure LLT and/or ALT the interferenceinteractions from the interference signal representing specularlyreflected light from the tear film produced by the imaging device can becompared to interference interactions in the 3-wave tear filminterference model.

The tear film interference model can be a theoretical tear filminterference model where the light source and the tear film layers aremodeled mathematically. The tear film layers may be mathematicallymodeled by modeling the tear film layers after certain biologicalmaterials. Interference interactions from the mathematically modeledlight source illuminating the mathematically modeled tear film andreceived by the mathematically modeled camera are calculated andrecorded for varying TFLTs. Alternatively, the tear film interferencemodel can be based on a biological or phantom tear film model comprisedof biological or phantom tear film layers. The actual light source isused to illuminate the biological or phantom tear film model andinterference interactions representing interference of specularlyreflected light are empirically observed and recorded for various TFLTsusing the actual camera.

Those skilled in the art will appreciate the scope of the presentdisclosure and realize additional aspects thereof after reading thefollowing detailed description of the preferred embodiments inassociation with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the disclosure, andtogether with the description serve to explain the principles of thedisclosure.

FIG. 1 is a side view of an exemplary eye showing the three layers ofthe tear film in exaggerated form;

FIG. 2 is a front view of an exemplary eye showing the lacrimal andaccessory tear glands that produce aqueous in the eye;

FIG. 3 illustrates exemplary upper and lower eyelids showing themeibomian glands contained therein;

FIGS. 4A through 4H illustrate relationships between light waves, beamsplitters, and polarizers;

FIG. 5 illustrates a general principle of operation for embodiments ofthe present disclosure;

FIGS. 6A and 6B illustrate an exemplary light source and imaging deviceto facilitate discussion of illumination of the tear film and capture ofinterference interactions of specularly reflected light from the tearfilm;

FIG. 7 illustrates (in a microscopic section view) exemplary tear filmlayers to illustrate how light rays can specularly reflect from varioustear film layer transitions;

FIG. 8 is a flowchart of an exemplary process that incorporatesalternating polarizers for obtaining one or more interference signalsfrom images of a tear film representing specularly reflected light fromthe tear film with background signal subtracted or substantiallysubtracted;

FIG. 9 illustrates an exemplary first image focused on a lipid layer ofa tear film and capturing interference interactions of specularlyreflected light from an area or region of interest of the tear film;

FIG. 10 illustrates an exemplary second image focused on the lipid layerof the tear film in FIG. 9 and capturing background signal whenilluminated by the light source;

FIG. 11 illustrates an exemplary image of the tear film when backgroundsignal captured in the second image of FIG. 10 is subtracted from thefirst image of FIG. 9;

FIG. 12 is a perspective view of an exemplary ocular surfaceinterferometry (OSI) device for illuminating and imaging a patient'stear film, displaying images, analyzing the patient's tear film, andgenerating results from the analysis of the patient's tear film;

FIG. 13 is a side view of the OSI device of FIG. 12 illuminating andimaging the eye and tear film;

FIG. 14A is a side view of an exemplary video camera and illuminatorwith a rotatable polarizer;

FIG. 14B is a top view of the video camera and illuminator in FIG. 14Awith the rotatable polarizer;

FIG. 15 is a flowchart of an exemplary process that employs a rotatablepolarizer for obtaining one or more interference signals from images ofa tear film representing specularly reflected light from the tear filmwith background signal subtracted or substantially subtracted;

FIG. 16 is a top view of the illumination device provided in the OSIdevice of FIGS. 14A and 14B illuminating the tear film with the videocamera capturing images of the patient's tear film;

FIG. 17 is a perspective view of an exemplary printed circuit board(PCB) with a plurality of light emitting diodes (LED) provided in theillumination device of the OSI device in FIGS. 14A and 14B to illuminatethe tear film;

FIG. 18 is a perspective view of the illumination device and housing inthe OSI device of FIGS. 14A and 14B;

FIG. 19A is a side view of an exemplary OSI device with a polarizerwheel having a plurality of polarizers;

FIG. 19B is a top view of the OSI device in FIG. 19A with the polarizerwheel having a plurality of polarizers;

FIG. 20 is a flowchart of an exemplary process that employs thepolarizer wheel in FIGS. 19A and 19B for obtaining one or moreinterference signals from images of a tear film representing specularlyreflected light from the tear film with background signal subtracted orsubstantially subtracted;

FIG. 21A is a side view of an exemplary OSI device employing a dualimager, beam splitter, and individual fixed polarizers configuration;

FIG. 21B is a top view of the OSI device in FIG. 21A in the dual imager,beam splitter, and individual fixed polarizers configuration;

FIG. 22 is a flow chart of an exemplary process for the OSI device thatincorporates the dual imager, beam splitter and individual fixedpolarizers configuration in FIGS. 21A and 21B;

FIG. 23A is a side view of an exemplary OSI device employing a dualimager and polarizing beam splitter configuration;

FIG. 23B is a top view of the OSI device in FIG. 23A employing the dualimager and polarizing beam splitter configuration;

FIG. 24A is a side view of an exemplary OSI device with the dual imagerand polarizing beam splitter configuration that also includes apolarizer in the illumination path of the illuminator;

FIG. 24B is a top view of the OSI device in FIG. 24A employing the dualimager and polarizing beam splitter that also includes a polarizer inthe illumination path of the illuminator;

FIG. 25A illustrates an exemplary system diagram of a control system andsupporting components that can include the exemplary OSI devices ofFIGS. 6A, 6B, 12, 13, 14A, 14B, 19A, 19B, 21A, 21B, 23A, 23B, 24A, and24B;

FIG. 25B is a flowchart illustrating an exemplary overall processingthat can be employed by the OSI devices of FIGS. 6A, 6B, 12, 13, 14A,14B, 19A, 19B, 21A, 21B, 23A, 23B, 24A, and 24B having systemscomponents according to the exemplary system diagram of the OSI devicein FIG. 25A;

FIG. 26 is a flowchart illustrating exemplary pre-processing stepsperformed on the combined first and second images of a patient's tearfilm before measuring tear film layer thickness (TFLT);

FIG. 27 is an exemplary graphical user interface (GUI) for controllingimaging, pre-processing, and post-processing settings that can beemployed by the OSI devices of FIGS. 6A, 6B, 12, 13, 14A, 14B, 19A, 19B,21A, 21B, 23A, 23B, 24A, and 24B;

FIG. 28 illustrates an example of a subtracted image in an area orregion of interest of a tear film containing specularly reflected lightfrom the tear film overlaid on top of a background image of the tearfilm;

FIGS. 29A and 29B illustrate exemplary threshold masks that may be usedto provide a threshold function during pre-processing of a resultingimage containing specularly reflected light from a patient's tear film;

FIG. 30 illustrates an exemplary image of FIG. 28 after a thresholdpre-processing function has been performed leaving interference of thespecularly reflected light from the patient's tear film;

FIG. 31 illustrates an exemplary image of the image of FIG. 30 aftererode and dilate pre-processing functions have been performed on theimage;

FIG. 32 illustrates an exemplary histogram used to detect eye blinksand/or eye movements in captured images or frames of a tear film;

FIG. 33 illustrates an exemplary process for loading an InternationalColour Consortium (ICC) profile and tear film interference model intothe OSI devices of FIGS. 6A, 6B, 12, 13, 14A, 14B, 19A, 19B, 21A, 21B,23A, 23B, 24A, and 24B;

FIG. 34 illustrates a flowchart providing an exemplary visualizationsystem process for displaying images of a patient's tear film on adisplay in the OSI devices of FIGS. 6A, 6B, 12, 13, 14A, 14B, 19A, 19B,21A, 21B, 23A, 23B, 24A, and 24B;

FIGS. 35A-35C illustrate exemplary images of a tear film with a patternof interference interactions from specularly reflected light from thetear film displayed on a display;

FIG. 36 illustrates an exemplary post-processing system that may beprovided in the OSI devices of FIGS. 6A, 6B, 12, 13, 14A, 14B, 19A, 19B,21A, 21B, 23A, 23B, 24A, and 24B;

FIG. 37A illustrates an exemplary 3-wave tear film interference modelbased on a 3-wave theoretical tear film model to correlate differentobserved interference color with different lipid layer thicknesses(LLTs) and aqueous layer thicknesses (ALTs);

FIG. 37B illustrates another exemplary 3-wave tear film interferencemodel based on a 3-wave theoretical tear film model to correlatedifferent observed interference color with different lipid layerthicknesses (LLTs) and aqueous layer thicknesses (ALTs);

FIG. 38 is another representation of the 3-wave tear film interferencemodel of FIG. 37 with normalization applied to each red-green-blue (RGB)color value individually;

FIG. 39 is an exemplary histogram illustrating results of a comparisonof interference interactions from the interference signal of specularlyreflected light from a patient's tear film to the 3-wave tear filminterference model of FIGS. 37 and 38 for measuring TFLT of a patient'stear film;

FIG. 40 is an exemplary histogram plot of distances in pixels betweenRGB color value representation of interference interactions from theinterference signal of specularly reflected light from a patient's tearfilm and the nearest distance RGB color value in the 3-wave tear filminterference model of FIGS. 37 and 38;

FIG. 41 is an exemplary threshold mask used during pre-processing of thetear film images;

FIG. 42 is an exemplary three-dimensional (3D) surface plot of themeasured LLT and ALT thicknesses of a patient's tear film;

FIG. 43 is an exemplary image representing interference interactions ofspecularly reflected light from a patient's tear film results windowbased on replacing a pixel in the tear film image with the closestmatching RGB color value in the normalized 3-wave tear film interferencemodel of FIG. 38;

FIG. 44 is an exemplary TFLT palette curve for a TFLT palette of LLTsplotted in RGB space for a given ALT in three-dimensional (3D) space;

FIG. 45 is an exemplary TFLT palette curve for the TFLT palette of FIG.44 with LLTs limited to a maximum LLT of 240 nm plotted in RGB space fora given ALT in three-dimensional (3D) space;

FIG. 46 illustrates the TFLT palette curve of FIG. 45 with an acceptabledistance to palette (ADP) filter shown to determine tear film pixelvalues having RGB values that correspond to ambiguous LLTs;

FIG. 47 is an exemplary login screen to a user interface system forcontrolling and accessing the OSI devices of FIGS. 6A, 6B, 12, 13, 14A,14B, 19A, 19B, 21A, 21B, 23A, 23B, 24A, and 24B;

FIG. 48 illustrates an exemplary interface screen for accessing apatient database interface in the OSI devices of FIGS. 6A, 6B, 12, 13,14A, 14B, 19A, 19B, 21A, 21B, 23A, 23B, 24A, and 24B;

FIG. 49 illustrates a patient action control box for selecting to eithercapture new tear film images of a patient in the patient database orview past captured images of the patient from the OSI devices of FIGS.6A, 6B, 12, 13, 14A, 14B, 19A, 19B, 21A, 21B, 23A, 23B, 24A, and 24B;

FIG. 50 illustrates a viewing interface for viewing a patient's tearfilm either captured in real-time or previously captured by the OSIdevices of FIGS. 6A, 6B, 12, 13, 14A, 14B, 19A, 19B, 21A, 21B, 23A, 23B,24A, and 24B;

FIG. 51 illustrates a tear film image database for a patient;

FIG. 52 illustrates a view images GUI screen showing an overlaid imageof interference interactions of the interference signals from specularlyreflected light from a patient's tear film overtop an image of thepatient's eye for both the patient's left and right eyes side by side;

FIG. 53 illustrates the GUI screen of FIG. 52 with the images of thepatient's eye toggled to show only the interference interactions of theinterference signals from specularly reflected light from a patient'stear film;

FIG. 54 is a flow chart that illustrates step-by-step processing of apolarization subtraction technique of the present disclosure; and

FIGS. 55A-F are examples of the images selected to implement thestep-by-step processing of FIG. 54.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the disclosure andillustrate the best mode of practicing the disclosure. Upon reading thefollowing description in light of the accompanying drawing figures,those skilled in the art will understand the concepts of the disclosureand will recognize applications of these concepts not particularlyaddressed herein. It should be understood that these concepts andapplications fall within the scope of the disclosure and theaccompanying claims.

Embodiments of the detailed description include ocular surfaceinterferometry (OSI) devices, systems, and methods for imaging an oculartear film and/or measuring a tear film layer thickness (TFLT) of anocular tear film. The OSI devices, systems, and methods can be used tomeasure the thickness of the lipid layer component (LLT) and/or theaqueous layer component (ALT) of the ocular tear film. “TFLT” as usedherein includes LLT, ALT, or both LLT and ALT. “Measuring TFLT” as usedherein includes measuring LLT, ALT, or both LLT and ALT. Imaging theocular tear film and measuring TFLT can be used in the diagnosis of apatient's tear film, including but not limited to lipid layer andaqueous layer deficiencies. These characteristics may be the cause orcontributing factor to a patient experiencing dry eye syndrome (DES).

In this regard, embodiments disclosed herein include a multi-wavelengthlight source that is controlled to direct light in the visible region toan ocular tear film. The light source may be a Lambertian emitter thatprovides a uniform or substantially uniform intensity in all directionsof emission. The light source is arranged such that light rays emittedfrom the light source are specularly reflected from the tear film andundergo constructive and destructive optical wave interferenceinteractions (also referred to as “interference interactions”) in theocular tear film. In some embodiments, the light source An imagingdevice having a detection spectrum that includes the spectrum of thelight source is focused on an area(s) of interest on the lipid layer ofthe tear film. The imaging device captures the interference interactions(i.e., modulation) of specularly reflected light rays from theilluminated tear film coming together by the focusing action of theimaging device in a first image. The imaging device then captures theoptical wave interference signals (also referred to as “interferencesignals”) representing the interference interactions of specularlyreflected light from the tear film. The imaging device produces anoutput signal(s) representative of the interference signal in a firstimage. The first image may contain an interference signal for a givenimaged pixel or pixels of the lipid layer by the imaging device.

The first image can be displayed to a technician or other user. Thefirst image can also be processed and analyzed to measure a TFLT in thearea or region of interest of the ocular tear film. In one embodiment,the first image also contains a background signal(s) that does notrepresent specularly reflected light from the tear film which issuperimposed on the interference signal(s). The first image is processedto subtract or substantially subtract out the background signal(s)superimposed upon the interference signal to reduce error before beinganalyzed to measure TFLT. This is referred to as “backgroundsubtraction” in the present disclosure. The separate backgroundsignal(s) includes returned captured light that is not specularlyreflected from the tear film and thus does not contain optical waveinterference information (also referred to as “interferenceinformation”). For example, the background signal(s) may include stray,ambient light entering into the imaging device, scattered light from theface and eye structures outside and within the tear film as a result ofambient light and diffuse illumination by the light source, and eyestructure beneath the tear film, and particularly contribution from theextended area of the source itself. The background signal(s) adds a bias(i.e., offset) error to the interference signal(s) thereby reducinginterference signal strength and contrast. This error can adverselyinfluence measurement of TFLT. Further, if the background signal(s) hasa color hue different from the light of the light source, a color shiftcan also occur to the captured optical wave interference (also referredto as “interference”) of specularly reflected light thus introducingfurther error.

In this regard in one embodiment, an apparatus for imaging an oculartear film is disclosed. The apparatus includes a control systemconfigured to receive at least one first image containing optical waveinterference of specularly reflecting light in a first polarizationplane along with a background signal from a region of interest (ROI) ofthe ocular tear film captured by an imaging device while illuminated bythe multi-wavelength light source. The control system is also configuredto receive at least one second image containing the background signal ina second polarization plane perpendicular or substantially perpendicularto the first polarization plane from the ROI of the ocular tear filmcaptured by an imaging device. In this manner, an imaging devicecaptures background signal(s) in a second image that is representativeof the signal which is superimposed on the interference of thespecularly reflecting light from the tear film in the first image. Thesecond image is subtracted from the first image to produce a resultingimage having isolated interference signal components. The resultingimage can then be displayed on a visual display to be analyzed by atechnician and/or processed and analyzed to measure a TFLT. Onenon-limiting benefit of the apparatus is that it allows capturing the atleast one second image containing the background signal in a secondpolarization plane perpendicular or substantially perpendicular to thefirst polarization plane from the ROI of the ocular tear film. As aresult, the background signal is isolated from the interference of thespecularly reflecting light from the tear film. Thus, a backgroundoffset captured in the at least one first image is removed or reducedfrom at least one resulting image generated by the subtraction of the atleast one second image from the at least one first image.

In another embodiment, a method of imaging an ocular tear film isdisclosed. The disclosed method involves illuminating the ROI of anocular tear film with the multi-wavelength light source. The methodinclude capturing optical wave interference of specularly reflectedlight in a first polarization plane including a background signal fromthe ROI of the ocular tear film while illuminated by themulti-wavelength light source in at least one image by an imagingdevice. The method also includes capturing the background signal in asecond polarizing plane perpendicular or substantially perpendicular tothe first polarization plane from the ROI of the ocular tear film in atleast one second image by an imaging device. The method also includessubtracting the at least one second image from the at least one firstimage to generate at least one resulting image containing the opticalwave interference of specularly reflected light from the ROI of theocular tear film with the background signal removed. Capturing the atleast one second image containing the background signal in a secondpolarization plane perpendicular or substantially perpendicular to thefirst polarization plane from the ROI of the ocular tear film canisolate the background signal from the interference of the specularlyreflecting light from the ocular tear film. In this manner, a backgroundoffset captured in the at least one first image is removed or reducedfrom at least one resulting image generated by the subtraction of the atleast one second image from the at least one first image.

Before discussing the particular embodiments of the present disclosure,a discussion of the electromagnetic nature of light waves is providedwith regard to FIGS. 4A-4H. Light waves, like all electromagnetic waves,propagate with an oscillating electric field that generates anoscillating magnetic field that regenerates the oscillating electricfield and so on. Moreover, light waves exhibit a property known aspolarization. For example, when the orientation of the oscillatingelectric field is in a fixed plane in space, the light wave is aplane-polarized wave. When the plane of the electric field rotates as alight wave propagates spirally through space, the light wave is acircularly or elliptically polarized light wave. Furthermore, light madeup of many light waves that are randomly polarized is referred to asunpolarized light, since there is no single polarization plane forunpolarized light.

Optical filters known as polarizers allow unimpeded transmission oflight waves having a single polarization-plane. Any type of polarizermay be employed in the embodiments discussed below. For example, alinear polarizer may be employed to reduce the intensity of a backgroundsignal(s). An exemplary linear polarizer has a polarization axis thattypically extends across the polarizer. Light waves that impinge uponthe linear polarizer with a polarization-plane that is parallel to thepolarization axis of the polarizer will pass through the polarizerunimpeded. In contrast, light waves that impinge upon the linearpolarizer with polarization-planes that are not parallel to thepolarization-plane of the polarizer will be impeded. As anothernon-limiting example, a circular polarizer or an elliptical polarizercan be employed in any of the embodiments below to reduce the intensityof a background signal(s). Circular polarizers and/or ellipticalpolarizers may also be usable to prevent unintentional secondaryspecular reflections in embodiments that use beam splitters. Anexemplary circular polarizer may comprise two components. One componentis a linear polarizer such as the exemplary polarizer described abovethat passes light waves in one polarization-plane. The other componentis a quarter wave plate that transforms light waves passing through thelinear polarizer in one polarization-plane into circularly polarizedlight waves. Exemplary elliptical polarizers may comprise the samecomponents as circular polarizers. An elliptical polarizer is configuredsuch that it transforms the light passing through the linear polarizerin one polarization-plane into elliptically polarized light.Elliptically polarized light has unequal electric field amplitudes.

The intensity of a light wave is related to its electric fieldamplitude. The degree to which the intensity of a light wave is reducedby a polarizer depends on the angle between the polarization plane ofthe light wave and the polarization axis of the polarizer. According toMalus' law, a light wave impinging on a polarizer will be reduced inamplitude in proportion to the cosine of the angle between thepolarization plane of the light wave and the polarization axis of thepolarizer. A light wave that has a polarization plane that is parallelwith the polarization axis will have a 0° angle between its polarizationplane and the polarization axis of the polarizer. Since the cosine of 0°is one, Malus' law ideally predicts no reduction of light intensity fora light wave that has a polarization plane that is parallel with thepolarization axis. In contrast, Malus' law ideally predicts notransmission through a polarizer for a light wave that impinges on thepolarizer with a polarization plane that is perpendicular to thepolarization axis of the polarizer since the cosine of 90° is zero.

When unpolarized light impinges on a polarizer, the overall intensity ofthe light is reduced by at least 50% due to the summations of amplitudereductions due to Malus' law as applied to individual light waves havingrandom polarization planes impinging on the polarizer. Light that istransmitted through the polarizer becomes polarized such that the lighthas a polarization plane that is parallel with the polarization axis ofthe polarizer.

Light intensity is proportional to the square of the amplitude.Therefore, the intensity of light transmitted through the polarizer froma non-polarized light source is ideally 25%. However, modern polarizersare not perfect, which results in additional losses of intensity for thetransmitted light due to absorption, scattering and other intensitydegrading effects.

In this regard, FIGS. 4A through 4H illustrate relationships betweenlight waves, polarizers and beam splitters. Referring to FIG. 4A, apolarizer 30 has a polarization axis 32 that is oriented in a directionindicated by a polarization axis arrow. A light wave 34 impinging on thepolarizer 30 is in a polarization plane that is parallel to thepolarization axis 32 of the polarizer 30. As a result, the light wave 34is transmitted unimpeded through the polarizer 30.

In contrast, FIG. 4B shows the polarizer 30 rotated such that thepolarization axis 32 is perpendicular to the polarization plane of thelight wave 34 as indicated by the polarization axis arrow. In this case,none of the light wave 34 will be transmitted through the polarizer 30.FIG. 4C and FIG. 4D represent the same conditions as FIG. 4A and FIG. 4Brespectively. However, the polarization plane of light wave 34 isrepresented by double headed arrows 36 instead of the sine waverepresentation used to represent light wave 34 in FIGS. 4A and 4B.

FIG. 4E illustrates a condition wherein the light wave 34 impinges uponthe polarizer 30 when the difference in the angle between thepolarization plane of the light wave 34 and the polarization axis 32 isintermediate of parallel and perpendicular. In this case, a portion ofthe light wave 34 is transmitted through the polarizer 30. The portionof light wave 34 that is transmitted through the polarizer 30 obtains apolarization plane that is parallel to the polarization axis 32 of thepolarizer 30. The intensity of the transmitted portion of the light wave34 is reduced relative to the intensity of the light wave 34 that isimpinging on the polarizer 30. The relative intensity of the light wave34 is represented by the relative difference in length of thedouble-headed arrows 36.

FIG. 4F illustrates a light beam 38 that is shown impinging on thepolarizer 30. The light beam 38 is made up of a plurality of light wavesthat are in random polarization planes. In this example, the light beam38 includes the light wave 34 having a polarization plane that alignswith the polarization axis 32 of the polarizer 30. As a result of thealignment, the light wave 34 is transmitted through the polarizer 30.

FIG. 4G illustrates a specular reflection and a refraction of the lightwave 34 as the light wave 34 impinges on the minor-like surface of apolarizing beam splitter 40. In the example of FIG. 4G, the light wave34 has a first polarization plane as it arrives at polarizing beamsplitter 40. A first portion 42 of the light wave 34 specularly reflectsfrom the polarizing beam splitter 40 in a second polarization plane thatis perpendicular to the first polarization plane of the impinging lightwave 34. A second portion 44 of the light wave 34 is refracted by thepolarizing beam splitter 40. The second portion 44 of the light wave 34maintains the first polarization plane of the impinging light wave 34.

FIG. 4H illustrates a specular reflection and a refraction of the lightwave 34 that is one of a plurality of light waves that make up the lightbeam 38 as it impinges upon the polarizing beam splitter 40. As in theprevious case, the light wave 34 in the first polarization plane issplit into the first portion 42 and the second portion 44. As before,the first portion 42 of the light wave 34 is specularly reflected fromthe polarizing beam splitter 40 in a polarization plane that isperpendicular to the first polarization plane of the impinging lightwave 34. As before, the second portion 44 of the light wave 34 maintainsthe first polarization plane of the impinging light wave 34.

FIG. 5 illustrates exemplary devices that can be provided in an OSIdevice according to a first embodiment. An optical system 46 is made upof a light source 48, a non-polarizing beam splitter 50, a firstpolarizer 52 having a polarization axis 54 and a second polarizer 56having a polarization axis 58. In the exemplary case of FIG. 5, light 60emitted from the light source 48 is unpolarized. However, the exemplarycase of FIG. 5 should not be viewed as limiting the present disclosureto unpolarized illumination, since at least one embodiment of thepresent disclosure utilizes a polarizer in the illumination path of alight source 48. The unpolarized light is directed from the light source48 to a target 62, which in the case of the present disclosure is a tearfilm covering the human eye. The target 62 reflects the unpolarizedlight to the non-polarizing beam splitter 50. After reflecting from thetarget 62, the unpolarized light 60 comprises an undesirable backgroundsignal that will be eliminated or reduced from images captured by theimaging device by processing, techniques discussed later in the presentdisclosure.

A portion of the unpolarized light 60 reflects from the non-polarizingbeam splitter 50 to the first polarizer 52. A portion of the unpolarizedlight 60 having a polarization plane that aligns with the polarizationaxis 54 passes through the first polarizer 52 unimpeded. A summation ofother light waves making up the unpolarized light 60 having polarizationplanes that are not perpendicular to the polarization axis 54 are alsotransmitted through the first polarizer 52 with reduced intensity. Lightwaves 64 that are transmitted through the first polarizer 52 each have apolarization plane that is parallel to the polarization axis 54.

A refracted portion of the unpolarized light 60 is directed to thesecond polarizer 56. Similar to the light interaction with the firstpolarizer 52, a portion of the unpolarized light 60 having apolarization plane that aligns with the polarization axis 58 passesthrough the second polarizer 56 unimpeded. A summation of other lightwaves making up the unpolarized light 60 having polarization planes thatare not perpendicular to the polarization axis 58 are also transmittedthrough the second polarizer 56 with reduced intensity. Light waves 66that are transmitted through the second polarizer 56 each have apolarization plane that is parallel to the polarization axis 58.

Another portion of the unpolarized light 60 emitted from the lightsource 48 is specularly reflected from the target 62 such that apolarized light 68 is directed to the non-polarizing beam splitter 50. Afirst portion of the polarized light 68 is specularly reflected from thenon-polarizing beam splitter 50 so that the polarized light isre-polarized in a polarization plane that is perpendicular to thepolarization axis 54 of the first polarizer 52. In this way, thepolarized light 68 that is specularly reflected from the target 62 ispractically prevented from passing through the first polarizer 52. Incontrast, a portion of the polarized light 68 that is refracted throughthe non-polarizing beam splitter 50 retains the polarization plane ofthe polarized light 68 as it specularly reflects from the target 62. Thepolarization axis 58 of the second polarizer 56 is aligned such that theportion of the specularly reflected light 84 passes through the secondpolarizer 56 unimpeded.

Against the discussion above, embodiments disclosed herein and discussedin more detail below include ocular surface interferometry (OSI)devices, systems, and methods for measuring a tear film layer thickness(TFLT) of the ocular tear film. The OSI devices, systems, and methodscan be used to measure the thickness of the lipid layer component (LLT)and/or the aqueous layer component (ALT) of the ocular tear film. “TFLT”as used herein includes LLT, ALT, or both LLT and ALT. “Measuring TFLT”as used herein includes measuring LLT, ALT, or both LLT and ALT.Measuring TFLT can be used in the diagnosis of an ocular tear film,including but not limited to lipid layer and aqueous layer deficiencies.These characteristics may be the cause or contributing factor to amammalian experiencing dry eye syndrome (DES).

In this regard, embodiments disclosed herein include a light source thatis controlled to direct light in the visible region to an ocular tearfilm. For example, the light source may be a Lambertian emitter thatprovides a uniform or substantially uniform intensity in all directionsof emission. The light source is arranged such that light rays emittedfrom the light source are specularly reflected toward an imaging devicefrom the tear film and undergo constructive and destructive interferenceinteractions in the ocular tear film. An imaging device having adetection spectrum that includes the spectrum of the light source isfocused on an area(s) of interest on the lipid layer of the tear film.The imaging device captures a first image of the interferenceinteractions (i.e., modulation) of specularly reflected light rays fromthe illuminated tear film coming together by the focusing action of theimaging device. The imaging device then captures the interferencesignals representing the interference interactions of specularlyreflected light from the tear film. The imaging device produces anoutput signal(s) representative of the interference signal in a firstimage. The first image may contain an interference signal for a givenimaged pixel or pixels of the lipid layer by the imaging device. Theoutput signal(s) can be processed and analyzed to measure a TFLT in thearea or region of interest of the ocular tear film.

OSI Device Employing Alternating Polarizers

In this regard, FIGS. 6A and 6B illustrate a general embodiment of anocular surface interferometry (OSI) device 70. Other embodiments will bedescribed later in this application. In general, the OSI device 70 isconfigured to illuminate a patient's ocular tear film, capture images ofinterference interactions of specularly reflected light from the oculartear film, and process and analyze the interference interactions tomeasure TFLT. As shown in FIG. 6A, the exemplary OSI device 70positioned in front of one of the patient's eye 72 is shown from a sideview. A top view of the patient 74 in front of the OSI device 70 isillustrated in FIG. 6B. The ocular tear film of a patient's eyes 72 isilluminated with a light source 76 (also referred to herein as“illuminator 76”) and comprises a large area light source having aspectrum in the visible region adequate for TLFT measurement andcorrelation to dry eye. The illuminator 76 can be a white ormulti-wavelength light source.

In this embodiment, the illuminator 76 is a Lambertian emitter and isadapted to be positioned in front of the eye 72 on a stand 78. Asemployed herein, the terms “Lambertian surface” and “Lambertian emitter”are defined to be a light emitter having equal or substantially equal(also referred to as “uniform” or substantially uniform) intensity inall directions. This allows the imaging of a uniformly or substantiallyuniformly bright tear film region for TFLT, as discussed in more detailin this disclosure. The illuminator 76 comprises a large surface areaemitter, arranged such that rays emitted from the emitter are specularlyreflected from the ocular tear film and undergo constructive anddestructive interference in tear film layers therein. An image of thepatient's 74 lipid layer is the backdrop over which the interferenceimage is seen and it should be as spatially uniform as possible.

An imaging device 80 is included in the OSI device 70 and is employed tocapture interference interactions of specularly reflected light from thepatient's 74 ocular tear film when illuminated by the illuminator 76.The imaging device 80 may be a still or video camera, or other devicethat captures images and produces an output signal representinginformation in captured images. The output signal may be a digitalrepresentation of the captured images.

Optical filtering is used to improve isolation of interferenceinteractions of specularly reflected light from the patient's 74 oculartear film before images of the patient's 74 ocular tear film arecaptured. In this regard, a first polarizer 52(1) is selectivelydisposable in an imaging path of the imaging device 80 during thecapture of at least one first image. A second polarizer 56(1) isselectively disposable in the imaging path of the imaging device 80during the capture of at least one second image.

When the first polarizer 52(1) is in front of imaging lens 82, thespecularly reflected light 84 from a region of interest (ROI) of theocular tear film is allowed to pass or substantially pass to the imagingdevice 80. Simultaneously, the background signal is reduced before thebackground signal reaches the imaging device 80. In contrast, when thesecond polarizer 56(1) is in front of imaging lens 82, the specularlyreflected light 84 from the ROI of the ocular tear film is reduced oreliminated before the specularly reflected light 84 reaches the imagingdevice 80 while passing a portion of the background signal. As a result,a first image captured with the first polarizer 52(1) in front of theimaging lens 82 will include a maximum amount of imagery resulting fromthe specularly reflected light 84 with a reduced amount of backgroundsignal. Moreover, a second image captured with the second polarizer56(1) in front of the imaging lens 82 will have a minimum amount or noneof the specularly reflected light along with a reduced backgroundsignal. Consequently, the second image can be subtracted from the firstimage to generate a resultant image that is predominately comprised ofthe ocular tear film.

As shown in FIGS. 6A and 6B, the first polarizer 52(1) and the secondpolarizer 56(1) are alternately translatable into the imaging path ofthe imaging device 80. In FIG. 6A the translation is shown occurringalong the Y-AXIS, whereas in FIG. 6B the translation is shown occurringalong the Z-AXIS. It is to be understood that translation of the firstpolarizer 52(1) and the second polarizer 56(1) along axes intermediatethe Y-AXIS and the Z-AXIS may also be available depending on clearancesand personal choice.

The geometry of the illuminator 76 can be understood by starting from animaging lens 82 of the imaging device 80 and proceeding forward to theeye 72 and then to the illuminator 76. The fundamental equation fortracing ray lines is Snell's law, which provides:n1 Sin Θ₁ =n2 Sin Θ₂,where “n1” and “n2” are the indexes of refraction of two mediumscontaining the ray, and Θ₁ and Θ₂ is the angle of the ray relative tothe normal from the transition surface. As illustrated in FIG. 7, lightrays 90 are directed by the illuminator 76 (FIGS. 6A and 6B) to anocular tear film 92. In the case of specularly reflected light 94 thatdoes not enter a lipid layer 96 and instead reflects from an anteriorsurface 98 of the lipid layer 96, Snell's law reduces down to Θ₁=Θ₂,since the index of refraction does not change (i.e., air in bothinstances). Under these conditions, Snell's law reduces to the classicallaw of reflection such that the angle of incidence is equal and oppositeto the angle of reflectance.

Some of the light rays 100 pass through the anterior surface 98 of thelipid layer 96 and enter into the lipid layer 96, as illustrated in FIG.7. As a result, the angle of these light rays 100 (i.e., Θ₃) normal tothe anterior surface 98 of the lipid layer 96 will be different than theangle of the light rays 90 (Θ₁) according to Snell's law. This isbecause the index of refraction of the lipid layer 96 is different thanthe index of refraction of air. Some of the light rays 100 passingthrough the lipid layer 96 will specularly reflect from the lipidlayer-to-aqueous layer transition 102 thereby producing specularlyreflected light rays 104. The specularly reflected light rays 94, 104undergo constructive and destructive interference anterior of the lipidlayer 96. The modulations of the interference of the specularlyreflected light rays 94, 104 superimposed on the anterior surface 98 ofthe lipid layer 96 are collected by the imaging device 80 when focusedon the anterior surface 98 of the lipid layer 96. Focusing the imagingdevice 80 on the anterior surface 98 of the lipid layer 96 allowscapturing of the modulated interference information at the plane of theanterior surface 98. In this manner, the captured interferenceinformation and the resulting calculated TFLT from the interferenceinformation is spatially registered to a particular area of the tearfilm 92 since that the calculated TFLT can be associated with suchparticular area, if desired.

The thickness of the lipid layer 96 (‘d₁’) is a function of theinterference interactions between specularly reflected light rays 94,104. The thickness of the lipid layer 96 (‘d₁’) is on the scale of thetemporal (or longitudinal) coherence of the light source 76. Therefore,thin lipid layer films on the scale of one wavelength of visible lightemitted by the light source 76 offer detectable colors from theinterference of specularly reflected light when viewed by a camera orhuman eye. The colors may be detectable as a result of calculationsperformed on the interference signal and represented as a digital valuesincluding but not limited to a red-green-blue (RGB) value in the RGBcolor space. Quantification of the interference of the specularlyreflected light can be used to measure LLT. The thicknesses of anaqueous layer 106 (‘d₂’) can also be determined using the sameprinciple. Some of the light rays 100 (not shown) passing through thelipid layer 96 can also pass through the lipid-to-aqueous layertransition 102 and enter into the aqueous layer 106 specularlyreflecting from the aqueous-to-mucin/cornea layer transition 108. Thesespecular reflections also undergo interference with the specularlyreflected light rays 94, 104. The magnitude of the reflections from eachinterface depends on the refractive indices of the materials as well asthe angle of incidence, according to Fresnel's equations, and so thedepth of the modulation of the interference interactions is dependent onthese parameters, thus so is the resulting color.

Turning back to FIGS. 6A and 6B, the illuminator 76 in this embodimentis a broad spectrum light source covering the visible region betweenabout 400 nm to about 700 nm. The illuminator 76 contains an arced orcurved housing 86 (see FIG. 6B) into which individual light emitters aremounted, subtending an arc of approximately 130 degrees from the opticalaxis of the eye 72 (see FIG. 6B). A curved surface may present betteruniformity and be more efficient, as the geometry yields a smallerdevice to generating a given intensity of light. The total powerradiated from the illuminator 76 should be kept to a minimum to preventaccelerated tear evaporation. Light entering the pupil can cause reflextearing, squinting, and other visual discomforts, all of which affectTFLT measurement accuracy.

In order to prevent alteration of the proprioceptive senses and reduceheating of the tear film 92, incident power and intensity on the eye 72may be minimized and thus, the step of collecting and focusing thespecularly reflected light may carried out by the imaging device 80. Theimaging device 80 may be a video camera, slit lamp microscope, or otherobservation apparatus mounted on the stand 78, as illustrated in FIGS.6A and 6B. Detailed visualization of the image patterns of the tear film92 involves collecting the specularly reflected light 84 and focusingthe specularly reflected light at the lipid layer 96 such that theinterference interactions of the specularly reflected light from theocular tear film are observable.

In operation, the first polarizer 52(1) is translated in front ofimaging lens 82 such that the polarization axis 54(1) is parallel orsubstantially parallel to the polarization plane of the specularlyreflected light 84 from a region of interest (ROI) of the ocular tearfilm. In this manner, the specularly reflected light 84 is allowed topass or substantially pass to the imaging device 80 while reducing thebackground signal before the background signal reaches the imagingdevice 80. Moreover, the second polarizer 56(1) is translated in frontof imaging lens 82 such that the polarization axis 58(1) isperpendicular or substantially perpendicular to the polarization planeof the specularly reflected light 84 from the ROI of the ocular tearfilm. In this manner, the specularly reflected light 84 is reduced oreliminated before the specularly reflected light 84 reaches the imagingdevice 80 while passing a portion of the background signal.

Against the backdrop of the OSI device 70 in FIGS. 6A and 6B, FIG. 8illustrates a flowchart discussing how the OSI device 70 can be used toobtain interference interactions of specularly reflected light from thetear film 92, which can be used to measure TFLT. Interferenceinteractions of specularly reflected light from the tear film 92 arefirst obtained and discussed before measurement of TFLT is discussed. Inthis embodiment as illustrated in FIG. 8, the process starts byadjusting the patient 74 with regard to an illuminator 76 and an imagingdevice 80 (block 110). The illuminator 76 is controlled to illuminatethe patient's 74 tear film 92. The imaging device 80 is controlled to befocused on the anterior surface 98 of the lipid layer 96 such that theinterference interactions of specularly reflected light from the tearfilm 92 are collected and are observable. Thereafter, the patient's 74tear film 92 is illuminated by the illuminator 76 (block 112).

The imaging device 80 is then controlled and focused on the lipid layer96 to collect specularly reflected light from an area or ROI on a tearfilm as a result of illuminating the tear film with the illuminator 76in a first image (block 114, FIG. 8). An example of the first image bythe illuminator 76 is provided in FIG. 9. As illustrated therein, afirst image 130 of a patient's eye 132 is shown that has beenilluminated with the illuminator 76. The illuminator 76 and the imagingdevice 80 may be controlled to illuminate an area or region of interest134 on a tear film 136 that does not include a pupil 138 of the eye 132so as to reduce reflex tearing. Reflex tearing will temporarily lead tothicker aqueous and lipid layers, thus temporarily altering theinterference signals of specularly reflected light from the tear film136. As shown in FIG. 9, when the imaging device 80 is focused on ananterior surface 142 of the lipid layer 144 of the tear film 136,interference interactions 140 of the interference signal of thespecularly reflected light from the tear film 136 as a result ofillumination by the illuminator 76 are captured in the area or region ofinterest 134 in the first image 130. The interference interactions 140appear to a human observer as colored patterns as a result of thewavelengths present in the interference of the specularly reflectedlight from the tear film 136.

However, even though the background signal is reduced by the firstpolarizer 52(1), a portion of the background signal is also captured inthe first image 130. The background signal is added to the specularlyreflected light in the area or region of interest 134 and includedoutside the area or region of interest 134 as well. Background signal islight that is not specularly reflected from the tear film 136 and thuscontains no interference information. Background signal can includestray and ambient light entering into the imaging device 80, scatteredlight from the patient's 74 face, eyelids, and/or eye 132 structuresoutside and beneath the tear film 136 as a result of stray light,ambient light and diffuse illumination by the illuminator 76, and imagesof structures beneath the tear film 136. For example, the first image130 includes the iris of the eye 132 beneath the tear film 136.Background signal adds a bias (i.e., offset) error to the capturedinterference of specularly reflected light from the tear film 136thereby reducing its signal strength and contrast. Further, if thebackground signal has a color hue different from the light of the lightsource, a color shift can also occur to the interference of specularlyreflected light from the tear film 136 in the first image 130. Theimaging device 80 produces a first output signal that represents thelight rays captured in the first image 130. Because the first image 130contains light rays from specularly reflected light as well as thebackground signal, the first output signal produced by the imagingdevice 80 from the first image 130 will contain an interference signalrepresenting the captured interference of the specularly reflected lightfrom the tear film 136 with a bias (i.e., offset) error caused by thebackground signal. As a result, the first output signal analyzed tomeasure TFLT may contain error as a result of the background signal bias(i.e., offset) error.

Thus, in this embodiment, the first output signal generated by theimaging device 80 as a result of the first image 130 is processed tosubtract or substantially subtract the background signal from theinterference signal to reduce error before being analyzed to measureTFLT. This is also referred to as “background subtraction.” Backgroundsubtraction is the process of removing unwanted reflections from images.In this regard, the imaging device 80 is controlled to capture a secondimage 146 of the tear film 136 as illustrated by example in FIG. 10.However, before the second image 146 is captured, a switch from thefirst polarizer 52(1) in the first orientation to the second polarizer56(1) in the second orientation is made (block 116 in FIG. 8). In thisway, the second image 146 will contain mostly background signal and whenthe second image 146 is subtracted from the first image 130 the capturedinterference of specularly reflected light from the tear film 136 willnot be reduced or at least not significantly reduced.

The second image 146 should be captured using the same imaging device 80settings and focal point as when capturing the first image 130 so thatthe first image 130 and second images 146 forms corresponding imagepairs captured within a short time of each other. The imaging device 80produces a second output signal containing background signal present inthe first image 130 (block 118 in FIG. 8). To eliminate or reduce thisbackground signal from the first output signal, the second output signalis subtracted from the first output signal to produce a resulting signal(block 120 in FIG. 8). The image representing the resulting signal inthis example is illustrated in FIG. 11 as resulting image 148. Thus, inthis example, background subtraction involves two images 130, 146 toprovide a frame pair where the two images 130, 146 are subtracted fromeach other, whereby specular reflection from the tear film 136 isretained, and while diffuse reflections from the iris and other areasare removed in whole or part.

As illustrated in FIG. 11, the resulting image 148 contains an image ofthe isolated interference 150 of the specularly reflected light from thetear film 136 with the background signal eliminated or reduced (block122 in FIG. 8). In this manner, the resulting signal (representing theresulting image 148 in FIG. 11) includes an interference signal havingsignal improved purity and contrast in the area or region of interest134 on the tear film 136. As will be discussed later in thisapplication, the resulting signal provides for accurate analysis ofinterference interactions from the interference signal of specularreflections from the tear film 136 to in turn accurately measure TFLT.Any method or device to obtain the first and second images of the tearfilm 136 and perform the subtraction of background signal in the secondimage 146 from the first image 130 may be employed. Other specificexamples are discussed throughout the remainder of this application.

An optional registration function may be performed between the firstimage(s) 130 and the second image(s) 146 before subtraction is performedto ensure that an area or point in the second image(s) 146 to besubtracted from the first image(s) 130 is for an equivalent orcorresponding area or point on the first image(s) 130. For example, aset of homologous points may be taken from the first and second images130, 146 to calculate a rigid transformation matrix between the twoimages. The transformation matrix allows one point on one image (e.g.,x1, y1) to be transformed to an equivalent two-dimensional (2D) image onthe other image (e.g., x2, y2). For example, the Matlab® function“cp2tform” can be employed in this regard. Once the transformationmatrix is determined, the transformation matrix can be applied to everypoint in the first and second images, and then each re-interpolated atthe original points. For example, the Matlab® function “imtransform” canbe employed in this regard. This allows a point from the second image(e.g., x2, y2) to be subtracted from the correct, equivalent point(e.g., x1, y1) on the first image(s) 130, in the event there is anymovement in orientation or the patient's eye between the capture of thefirst and second images 130, 146. The first and second images 130, 146should be captured close in time. A similar registration technique isexplained in greater detail later on in this disclosure.

Note that while this example discusses a first image and a second imagecaptured by the imaging device 80 and a resulting first output signaland second output signal, the first image and the second image maycomprise a plurality of images taken in a time-sequenced fashion. If theimaging device 80 is a video camera, the first and second images maycontain a number of sequentially-timed frames governed by the frame rateof the imaging device 80. The imaging device 80 produces a series offirst output signals and second output signals. If more than one imageis captured, the subtraction performed in a first image should ideallybe from a second image taken immediately after the first image so thatthe same or substantially the same lighting conditions exist between theimages so the background signal in the second image is present in thefirst image.

The subtraction of the second output signal from the first output signalcan be performed in real time. Alternatively, the first and secondoutput signals can be recorded and processed at a later time. Theilluminator 76 may be controlled to oscillate off and on quickly so thatfirst and second images can be taken and the second output signalsubtraction from the first output signal be performed in less than onesecond. For example, if the illuminator 76 oscillates between on and offat 30 Hz, the imaging device 80 can be synchronized to capture images ofthe tear film 92 at 106 frames per second (fps). In this regard, thirty(30) first images and thirty (30) second images can be obtained in onesecond, with each pair of first and second images taken sequentially.

After the interference of the specularly reflected light is captured anda resulting signal containing the interference signal is produced andprocessed, the interference signal or representations thereof can becompared against a tear film layer interference model to measure TFLT.The interference signal can be processed and converted by the imagingdevice into digital red-green-blue (RGB) component values which can becompared to RGB component values in a tear film interference model tomeasure tear film TFLT. The tear film interference model is based onmodeling the lipid layer of the tear film in various LLTs andrepresenting resulting interference interactions in the interferencesignal of specularly reflected light from the tear film model whenilluminated by the light source. The tear film interference model can bea theoretical tear film interference model where the particular lightsource, the particular imaging device, and the tear film layers aremodeled mathematically, and the resulting interference signals for thevarious LLTs recorded when the modeled light source illuminates themodeled tear film layers recorded using the modeled imaging device. Thesettings for the mathematically modeled light source and imaging deviceshould be replicated in the illuminator 76 and imaging device 80 used inthe OSI device 70. Alternatively, the tear film interference model canbe based on a phantom tear film model, comprised of physical phantomtear film layers wherein the actual light source is used to illuminatethe phantom tear film model and interference interactions in theinterference signal representing interference of specularly reflectedlight are empirically observed and recorded using the actual imagingdevice.

The aqueous layer may be modeled in the tear film interference model tobe of an infinite, minimum, or varying thickness. If the aqueous layeris modeled to be of an infinite thickness, the tear film interferencemodel assumes no specular reflections occur from the aqueous-to-mucinlayer transition 108 (see FIG. 7). If the aqueous layer 106 is modeledto be of a certain minimum thickness (e.g., ≧2 μm), the specularreflection from the aqueous-to-mucin layer transition 108 may beconsidered negligible on the effect of the convolved RGB signalsproduced by the interference signal. In either case, the tear filminterference model will only assume and include specular reflectionsfrom the lipid-to-aqueous layer transition 102. Thus, these tear filminterference model embodiments allow measurement of LLT regardless ofALT. The interference interactions in the interference signal arecompared to the interference interactions in the tear film interferencemodel to measure LLT.

Alternatively, if the aqueous layer 106 is modeled to be of varyingthicknesses, the tear film interference model additionally includesspecular reflections from the aqueous-to-mucin layer transition 108 inthe interference interactions. As a result, the tear film interferencemodel will include two-dimensions of data comprised of interferenceinteractions corresponding to various LLT and ALT combinations. Theinterference interactions from the interference signal can be comparedto interference interactions in the tear film interference model tomeasure both LLT and ALT. More information regarding specific tear filminterference models will be described later in this application.

In the above described embodiment in FIGS. 8-11, the second image 146 ofthe tear film 136 contains a background signal. Only ambient lightilluminates the tear film 136 and eye 132 structures beneath. Thus, thesecond image 146 and the resulting second output signal produced by theimaging device 80 from the second image 146 does not include backgroundsignal resulting from scattered light from the patient's face and eyestructures as a result of diffuse illumination by the illuminator 76.Only scattered light resulting from ambient light is included in thesecond image 146. However, scattered light resulting from diffuseillumination by the illuminator 76 is included in background signal inthe first image 130 containing the interference interactions ofspecularly reflected light from the tear film 136.

Further, because the first image 130 is captured when the illuminator 76is illuminating the tear film, the intensity of the eye structuresbeneath the tear film 136 captured in the first image 130, including theiris, are brighter than captured in the second image 146. Thus, in otherembodiments described herein, the imaging device 80 is controlled tocapture a second image of the tear film 136 when obliquely illuminatedby the illuminator 76. As a result, the captured second imageadditionally includes background signal from scattered light as a resultof diffuse illumination by the illuminator 76 as well as a higherintensity signal of the eye directly illuminated structures beneath thetear film 136. Thus, when the second output signal is subtracted fromthe first output signal, the higher intensity eye structure backgroundand the component of background signal representing scattered light as aresult of diffuse illumination by the illuminator 76, as well as ambientand stray light, are subtracted or substantially subtracted from theresulting signal thereby further increasing the interference signalpurity and contrast in the resulting signal. The resulting signal canthen be processed and analyzed to measure TFLT, as will be described indetail later in this application.

Exemplary OSI Devices

The above discussed illustrations provide examples of illuminating andimaging a patient's TFLT. These principles are described in more detailwith respect to a specific example of an OSI device 170 illustrated inFIGS. 12-55F and described below throughout the remainder of thisapplication. The OSI device 170 can illuminate a patient's tear film,capture interference information from the patient's tear film, andprocess and analyze the interference information to measure TFLT.Further, the OSI device 170 includes a number of optional pre-processingfeatures that may be employed to process the interference signal in theresulting signal to enhance TFLT measurement. The OSI device 170 mayinclude a display and user interface to allow a physician or technicianto control the OSI device 170 to image a patient's eye and tear film andmeasure the patient's TFLT.

Illumination and Imaging

In this regard, FIG. 12 illustrates a perspective view of the OSI device170. The OSI device 170 is designed to facilitate imaging of thepatient's ocular tear film and processing and analyzing the images todetermine characteristics regarding a patient's tear film. The OSIdevice 170 includes an imaging device and light source in this regard,as will be described in more detail below. As illustrated in FIG. 12,the OSI device 170 is comprised generally of a housing 172, a displaymonitor (“display”) 174, and a patient head support 176. The housing 172may be designed for table top placement. The housing 172 rests on a base178 in a fixed relationship. As will be discussed in more detail below,the housing 172 houses an imaging device and other electronics,hardware, and software to allow a clinician to image a patient's oculartear film. A light source 173 (also referred to herein as “illuminator173”) is also provided in the housing 172 and provided behind adiffusing translucent window 175. The translucent window 175 may be aflexible, white, translucent acrylic plastic sheet.

To image a patient's ocular tear film, the patient places his or herhead in the patient head support 176 and rests his or her chin on a chinrest 180. The chin rest 180 can be adjusted to align the patient's eyeand tear film with the imaging device inside the housing 172, as will bediscussed in more detail below. The chin rest 180 may be designed tosupport up to two (2) pounds of weight, but such is not a limitingfactor. A transparent window 177 allows the imaging device inside thehousing 172 to have a clear line of sight to a patient's eye and tearfilm when the patient's head is placed in the patient head support 176.The first polarizer 52(1) shown in dashed line is behind the transparentwindow 177 and within the imaging path of the imaging device (notvisible). The second polarizer 56(1) is also shown in dashed line.However, in this example the second polarizer 56(1) is shown translatedaway from the imaging path of the imaging device. The OSI device 170 isdesigned to image one eye at a time, but can be configured to image botheyes of a patient, if desired.

In general, the display 174 provides input and output from the OSIdevice 170. For example, a user interface can be provided on the display174 for the clinician to operate the OSI device 170 and to interact witha control system provided in the housing 172 that controls the operationof the OSI device 170, including an imaging device, an imaging devicepositioning system, a light source, other supporting hardware andsoftware, and other components. For example, the user interface canallow control of imaging positioning, focus of the imaging device, andother settings of the imaging device for capturing images of a patient'socular tear film. The control system may include a general purposemicroprocessor or computer with memory for storage of data, includingimages of the patient's eye and tear film. The microprocessor should beselected to provide sufficient processing speed to process images of thepatient's tear film and generate output characteristic information aboutthe tear film (e.g., one minute per twenty second image acquisition).The control system may control synchronization of activation of thelight source and the imaging device to capture images of areas ofinterest on the patient's ocular tear film when properly illuminated.Various input and output ports and other devices can be provided,including but not limited to a joystick for control of the imagingdevice, USB ports, wired and wireless communication including Ethernetcommunication, a keyboard, a mouse, speaker(s), etc. A power supply isprovided inside the housing 172 to provide power to the componentstherein requiring power. A cooling system, such as a fan, may also beprovided to cool the OSI device 170 from heat generating componentstherein.

The display 174 is driven by the control system to provide informationregarding a patient's imaged tear film, including TFLT. The display 174also provides a graphical user interface (GUI) to allow a clinician orother user to control the OSI device 170. To allow for human diagnosisof the patient's tear film, images of the patient's ocular tear filmtaken by the imaging device in the housing 172 can also be displayed onthe display 174 for review by a clinician, as will be illustrated anddescribed in more detail below. The images displayed on the display 174may be real-time images being taken by the imaging device, or may bepreviously recorded images stored in memory. To allow for differentorientations of the OSI device 170 to provide a universal configurationfor manufacturing, the display 174 can be rotated about the base 178.The display 174 is attached to a monitor arm 182 that is rotatable aboutthe base 178, as illustrated. The display 174 can be placed opposite ofthe patient head support 176, as illustrated in FIG. 12, if theclinician desires to sit directly across from the patient.Alternatively, display 174 can be rotated either left or right about theX-axis to be placed adjacent to the patient head support 176. Thedisplay 174 may be a touch screen monitor to allow a clinician or otheruser to provide input and control to the control system inside thehousing 172 directly via touch of the display 174 for control of the OSIdevice 170. The display 174 illustrated in FIG. 12 is a fifteen inch(15″) flat panel liquid crystal display (LCD). However, the display 174may be provided of any type or size, including but not limited to acathode ray tube (CRT), plasma, LED, OLED, projection system, etc.

FIG. 13 illustrates a side view of the OSI device 170 of FIG. 12 tofurther illustrate imaging of a patient's eye and ocular tear film. Asillustrated therein, a patient places their head 184 in the patient headsupport 176. More particularly, the patient places their forehead 186against a headrest 188 provided as part of the patient head support 176.The patient places their chin 190 in the chin rest 180. The patient headsupport 176 is designed to facilitate alignment of a patient's eye 192with the OSI device 170, and in particular, an imaging device 194 (andilluminator) shown as being provided inside the housing 172. The chinrest 180 can be adjusted higher or lower to move the patient's eye 192with respect to the OSI device 170. The first polarizer 52(1) shown indashed line is within the imaging path of the imaging device 194. Thesecond polarizer 56(1) is also shown in dashed line. However, in thisexample the second polarizer 56(1) is shown translated below the firstpolarizer 52(1) and away from the imaging path of the imaging device.

OSI Device Employing Rotatable Polarizer

As shown in FIGS. 14A and 14B, the imaging device 194 can be used toimage the patient's ocular tear film to determine characteristics of thepatient's tear film. In particular, the imaging device 194 is used tocapture interference interactions of the specularly reflected light fromthe patient's tear film when illuminated by the light source 196 (alsoreferred to herein as “illuminator 196”) as well as background signal.In the OSI device 170, the imaging device 194 is the “The ImagingSource” model DFK21BU04 charge coupling device (CCD) digital videocamera 198, but many types of metrological grade cameras or imagingdevices can be provided. A CCD camera enjoys characteristics ofefficient light gathering, linear behavior, cooled operation, andimmediate image availability. A linear imaging device is one thatprovides an output signal representing a captured image which isprecisely proportional to the input signal from the captured image.Thus, use of a linear imaging device (e.g., gamma correction set to 1.0,or no gamma correction) provides undistorted interference data which canthen be analyzed using linear analysis models. In this manner, theresulting images of the tear film do not have to be linearized beforeanalysis, thus saving processing time. Gamma correction can then beadded to the captured linear images for human-perceptible display on thenon-linear display 174 in the OSI device 170 (FIGS. 12 and 13).Alternatively, the opposite scenario could be employed. That is, anon-linear imaging device or non-linear setting would be provided tocapture tear film images, wherein the non-linear data representing theinterference interactions of the interference signal can be provided toa non-linear display monitor without manipulation to display the tearfilm images to a clinician. The non-linear data would be linearized fortear film processing and analysis to estimate tear film layer thickness.

The video camera 198 is capable of producing lossless full motion videoimages of the patient's eye. As illustrated in FIGS. 14A and 14B, thevideo camera 198 has a depth of field defined by the angle between rays199 and the lens focal length that allows the patient's entire tear filmto be in focus simultaneously. The video camera 198 has an externaltrigger support so that the video camera 198 can be controlled by acontrol system to image the patient's eye. The video camera 198 includesa lens and a rotatable polarizer 208 that can fit within the housing 172(FIG. 13) that is usable in place of both the first and secondpolarizers 52(1) and 56(1).

Employing the rotatable polarizer 208 is less mechanically complex thanalternating the first and second polarizers 52(1) and 56(1) into theimaging path of the video camera 198. For example, the rotatablepolarizer 208 is always in the imaging path of the video camera 198.Therefore, the rotatable polarizer 208 only needs to be rotated from onepolarization orientation to another between a first and second imagecapture. Moreover, due to its relatively low mechanical complexity, therotatable polarizer 208 can offer a relatively fast response whenrotating from one polarization orientation to another. This relativelyfast response allows the rotatable polarizer 208 to be more easilysynchronized with the video camera 198.

The rotatable polarizer 208 is disposed in an imaging path of theimaging device 194, which in this case is the video camera 198. Therotatable polarizer 208 has a center axis shown in dashed line extendingto the eye 192. The rotatable polarizer 208 is selectively rotatableabout the center axis to provide a first polarization axis relative tothe polarization plane of the specularly reflected light from the ROI ofthe ocular tear film on the eye 192 during a capture of at least onefirst image. The rotatable polarizer 208 is also selectively rotatableto provide a second polarization axis that is perpendicular orsubstantially perpendicular relative to the first polarization axisduring the capture of at least one second image. An actuator such as amotor could be coupled to the rotatable polarizer 208 to rotate the fromone polarization orientation to another in synchronization with thevideo camera 198.

The video camera 198 in this embodiment has a resolution of 640×480pixels and is capable of frame rates up to sixty (60) frames per second(fps). The lens system employed in the video camera 198 images a 16×12mm dimension in a sample plane onto an active area of a CCD detectorwithin the video camera 198. As an example, the video camera 198 may bethe DBK21AU04 Bayer VGA (640×480) video camera using a Pentax VS-LD25Daitron 25-mm fixed focal length lens. Other camera models withalternate pixel size and number, alternate lenses, (etc) may also beemployed.

Although a video camera 198 is provided in the OSI device 170, a stillcamera could also be used if the frame rate is sufficiently fast enoughto produce high quality images of the patient's eye. High frame rate inframes per second (fps) facilitate high quality subtraction ofbackground signal from a captured interference signal representingspecularly reflected light from a patient's tear film, and may provideless temporal (i.e., motion) artifacts (e.g., motion blurring) incaptured images, resulting in high quality captured images. This isespecially the case since the patient's eye may move irregularly as wellas blinking, obscuring the tear film from the imaging device duringexamination.

A camera positioning system 200 is also provided in the housing 172 ofthe OSI device 170 to position the video camera 198 for imaging of thepatient's tear film. The camera positioning system 200 is under thecontrol of a control system. In this manner, a clinician can manipulatethe position of the video camera 198 to prepare the OSI device 170 toimage the patient's tear film. The camera positioning system 200 allowsa clinician and/or control system to move the video camera 198 betweendifferent patients' eyes 192, but can also be designed to limit therange of motion within designed tolerances. The camera positioningsystem 200 also allows for fine tuning of the video camera 198 position.The camera positioning system 200 includes a stand 202 attached to abase 204. A linear servo or actuator 206 is provided in the camerapositioning system 200 and connected between the stand 202 and a cameraplatform 207 supporting the video camera 198 to allow the video camera198 to be moved in the vertical (i.e., Y-axis) direction.

In this embodiment of the OSI device 170, the camera positioning system200 may not allow the video camera 198 to be moved in the X-axis or theZ-axis (in and out of FIG. 14A), but other embodiments of the disclosureare not so limited. The illuminator 196 is also attached to the cameraplatform 207 such that the illuminator 196 maintains a fixed geometricrelationship to the video camera 198. Thus, when the video camera 198 isadjusted to the patient's eye 192, the illuminator 196 is automaticallyadjusted to the patient's eye 192 in the same regard as well. This maybe important to enforce a desired distance (d) and angle of illumination(Φ) of the patient's eye 192, as illustrated in FIG. 14A, to properlycapture the interference interactions of the specularly reflected lightfrom the patient's tear film at the proper angle of incidence accordingto Snell's law, since the OSI device 170 is programmed to assume acertain distance and certain angles of incidence. In the OSI device 170in FIG. 14A, the angle of illumination (Φ) of the patient's eye 192relative to the video camera 198 axis is approximately 30 degrees at thecenter of the illuminator 196 and includes a relatively large range ofangles from about 5 to 60 degrees, but any angle may be provided.

FIG. 15 is a flowchart of an exemplary process for OSI device 170 thatincorporates the rotatable polarizer 208 for obtaining one or moreinterference signals from images of a tear film representing specularlyreflected light from the tear film with background signal subtracted orsubstantially subtracted. Against the backdrop of the OSI device 170 inFIGS. 14A and 14B, FIG. 15 illustrates a flowchart discussing how theOSI device 170 can be used to obtain interference interactions ofspecularly reflected light from the tear film 92, which can be used tomeasure TFLT. Interference interactions of specularly reflected lightfrom the tear film 92 are first obtained and discussed beforemeasurement of TFLT is discussed. In this embodiment as illustrated inFIG. 15, the process starts by adjusting the patient 184 with regard tothe illuminator 196 and the imaging device 194 (block 210). Theilluminator 196 is then controlled to illuminate the patient's 184 tearfilm 92(block 212).

Next, the imaging device 194 is controlled to be focused on the anteriorsurface 98 of the lipid layer 96 such that the interference interactionsof specularly reflected light from the tear film 92 are captured infirst image(s) (block 214). An example of the first image(s) captured bythe imaging device is provided in FIG. 9. As illustrated therein, afirst image 130 of a patient's eye 192 is shown that has beenilluminated with the illuminator 196. The illuminator 196 and theimaging device 194 may be controlled to illuminate an area or region ofinterest 134 on a tear film 136 that does not include a pupil 138 of theeye 132 so as to reduce reflex tearing. Reflex tearing will temporarilylead to thicker aqueous and lipid layers, thus temporarily altering theinterference signals of specularly reflected light from the tear film136. As shown in FIG. 9, when the imaging device 194 is focused on ananterior surface 142 of the lipid layer 144 of the tear film 136,interference interactions 140 of the interference signal of thespecularly reflected light from the tear film 136 as a result ofillumination by the illuminator 76 are captured in the area or region ofinterest 134 in the first image 130. The interference interactions 140appear to a human observer as colored patterns as a result of thewavelengths present in the interference of the specularly reflectedlight from the tear film 136.

However, even though the background signal is reduced by the rotatablepolarizer 208, a portion of the background signal is also captured inthe first image 130. The background signal is added to the specularlyreflected light in the area or region of interest 134 and includedoutside the area or region of interest 134 as well. Background signal islight that is not specularly reflected from the tear film 136 and thuscontains no interference information. Background signal can includestray and ambient light entering into the imaging device 194, scatteredlight from the patient's 184 face, eyelids, and/or eye 192 structuresoutside and beneath the tear film 136 as a result of stray light,ambient light and diffuse illumination by the illuminator 196, andimages of structures beneath the tear film 136. For example, the firstimage 130 includes the iris of the eye 132 beneath the tear film 136.Background signal adds a bias (i.e., offset) error to the capturedinterference of specularly reflected light from the tear film 136thereby reducing its signal strength and contrast. Further, if thebackground signal has a color hue different from the light of the lightsource, a color shift can also occur due to the interference ofspecularly reflected light from the tear film 136 in the first image130.

The imaging device 194 produces a first output signal that representsthe light rays captured in the first image 130. Because the first image130 contains light rays from specularly reflected light as well as thebackground signal, the first output signal produced by the imagingdevice 194 from the first image 130 will contain an interference signalrepresenting the captured interference of the specularly reflected lightfrom the tear film 136 with a bias (i.e., offset) error caused by thebackground signal. As a result, the first output signal analyzed tomeasure TFLT may contain error as a result of the background signal bias(i.e., offset) error.

Thus, in this embodiment, the first output signal generated by theimaging device 194 as a result of the first image 130 is processed tosubtract or substantially subtract the background signal from theinterference signal to reduce error before being analyzed to measureTFLT. This is also referred to as “background subtraction.” Backgroundsubtraction is the process of removing unwanted reflections from images.In this regard, the imaging device 194 is controlled to capture a secondimage 146 of the tear film 136. However, before the second image 146 iscaptured, the rotatable polarizer 208 is rotated from a firstorientation that reduces background signal to a second orientation thatreduces specularly reflected light (block 216 in FIG. 15). In this way,the second image 146 will contain mostly background signal and when thesecond image 146 is subtracted from the first image 130 the capturedinterference of specularly reflected light from the tear film 136 willnot be reduced or at least not significantly reduced.

The second image 146 should be captured using the same imaging device194 settings and focal point as when capturing the first image 130 sothat the first image 130 and second image 146 form corresponding imagepairs captured within a short time of each other. The imaging device 194produces a second output signal containing background signal present inthe first image 130 (block 218 in FIG. 15). To eliminate or reduce thisbackground signal from the first output signal, the second output signalis subtracted from the first output signal to produce a resulting signal(block 220 in FIG. 15). The image(s) representing the resultingsignal(s) containing interference signal(s) of specularly reflectedlight from the tear film is produced (block 222 in FIG. 15). In thisexample the resulting image 148 is illustrated in FIG. 11. Thus, in thisexample, background subtraction involves two images 130, 146 to providea frame pair where the two images 130, 146 are subtracted from eachother, whereby specular reflection from the tear film 136 is retained,and while diffuse reflections from the iris and other areas are removedin whole or part.

Illuminator Details

FIGS. 16-18 provide more detail on the illuminator 196. As illustratedin FIG. 16, the exemplary illuminator 196 is provided on an arcedsurface 230 (see also, FIGS. 17-18) of approximately 75 degrees toprovide a large area, broad spectrum light source covering the visibleregions of approximately 400 nanometers (nm) to 700 nm. In thisembodiment, the arced surface 230 has a radius to an imaginary center ofapproximately 190 mm (“r” in FIG. 16) and has a face 250 mm high by 100mm wide. The arced surface 230 could be provided as a flat surface, butan arced surface may allow for: better illumination uniformity, and, asmaller sized illuminator 196 for packaging constraints, while providingthe same effective illumination area capability. In this example, theilluminator 196 is a Lambertian emitter wherein the light emitter hasapproximately the same intensity in all directions; however, otherembodiments of the present disclosure are not so limited. Theilluminator 196 is arranged so that, from the perspective of the videocamera 198, emitted light rays are specularly reflected from the tearfilm of the patient's eye 192 and undergo constructive and destructiveinterference in the lipid layer and layers beneath the lipid layer. Inthis embodiment, the illuminator 196 is comprised of high efficiency,white light emitting diodes (LEDs) 232 (see FIGS. 17 and 18) mounted ona printed circuit board (PCB) 236 (FIG. 17). Supporting circuitry (notshown) may be included to control operation of the LEDs 232, and toautomatically shut off the LEDs 232 when the OSI device 170 is not inuse. Each LED 232 has a 120 degree (“Lambertian”) forward projectionangle, a 1350 mcd maximum intensity, manufactured by LEDtronics. Otherlight sources other than LEDs are also possible, including but notlimited to lasers, incandescent light, and organic LEDs (OLEDs), asexamples. Further, the light source is not required to be a Lambertianemitter. For example, the light emitted from the light source may becollimated.

As illustrated in FIG. 18, the PCB 236 is placed inside an illuminatorhousing 242. The illuminator housing 242 is comprised of two side panels244A, 244B that are disposed on opposite sides of the arced surfaced 230when held by base panel 246 and top panel 248, and also includes a rearpanel 234. The arced surface 230 is comprised of a diffuser 240 todiffuse the light emitted by the LEDs 232. The diffuser 240 can beselected to minimize intensity reduction, while providing sufficientscattering to make the illumination uniform light wave fall off on thelight emitted by the outside LEDs 232. The diffuser 240, PCB 236, andrear panel 234 are flexible and fit within grooves 250 located in thetop panel 248 and base panel 246, and grooves 252 located in the sidepanels 244A, 244B. The illuminator housing 242 is snapped together andthe side panels 244A, 244B are then screwed to the top panel 248 andbase panel 246.

The diffuser 240 may also be comprised of more than one diffuser panelto improve uniformity in the light emitted from the illuminator 196. Theside panels 244A, 244B and the base panel 246 and top panel 248 formbaffles around the PCB 236 and the LEDs 232. The inside of thesesurfaces may contain a reflective film (e.g., 3M ESR film) to assist inthe uniformity of light emitted by the LEDs 232. The reflective film mayassist in providing a uniform light intensity over an entire area orregion of interest on a patient's tear film. This may be particularly anissue on the outer edges of the illumination pattern. The diffuser 240should also be sufficiently tightly held to the edges in the illuminatorhousing 242 to prevent or reduce shadows on in the illumination pattern.

OSI Device Employing Polarizer Wheel

FIG. 19A is a side view of the imaging device 194 and illuminator 196configured with a polarizer wheel 260 having a plurality of polarizers.FIG. 19B is a top view of the imaging device 194 and illuminator 196with the polarizer wheel 260. In particular, the polarizer wheel 260includes a plurality of polarizers made up of a first polarizer 52(2)and a second polarizer 56(2). The polarizer wheel 260 is selectivelyrotatable such that the first polarizer 52(2) is disposed in an imagingpath of the imaging device 194 during the capture of the at least onefirst image, and wherein the second polarizer 56(2) is alternatelydisposed in the imaging path of the imaging device 194 during thecapture of the at least one second image. In this manner, the firstpolarizer 52(2) and the second polarizer 56(2) are alternately rotatableinto the imaging path of the imaging device 194 that includes rays 199and is centered on the dashed line that extends from eye 192. A motor262 coupled to polarizer wheel 260 is controllable to selectivelysynchronize the rotation of the polarizer wheel 260 with the imagecapturing by the imaging device 194.

The first polarizer 52(2) has a polarization axis 54(2) that is parallelor substantially parallel to the polarization plane of the specularlyreflected light from the ROI of the ocular tear film 92 (FIG. 7) to passor substantially pass the specularly reflected light to the imagingdevice 194 while reducing the background signal. Furthermore, the secondpolarizer 56(2) has a polarization axis 58(2) that is perpendicular orsubstantially perpendicular to the polarization plane of the specularlyreflected light from the ROI of the ocular tear film 92 (FIG. 7) toeliminate or substantially eliminate the specularly reflected lightwhile passing a portion of background signal to the imaging device 194.

FIG. 20 is a flowchart of an exemplary process for the OSI device 170that incorporates the polarizer wheel 260 for obtaining one or moreinterference signals from images of a tear film 92 representingspecularly reflected light from the tear film 92 with background signalsubtracted or substantially subtracted. Against the backdrop of the OSIdevice 170 in FIGS. 19A and 19B, FIG. 20 illustrates a flowchartdiscussing how the OSI device 170 can be used to obtain interferenceinteractions of specularly reflected light from the tear film 92, whichcan be used to measure TFLT. Interference interactions of specularlyreflected light from the tear film 92 are first obtained and discussedbefore measurement of TFLT is discussed. In this embodiment asillustrated in FIG. 20, the process starts by adjusting the patient 184with regard to the illuminator 196 and the imaging device 194 (block270). The illuminator 196 is controlled to illuminate the patient's 184tear film 92. The imaging device 194 is controlled to be focused on theanterior surface 98 of the lipid layer 96 such that the interferenceinteractions of specularly reflected light from the tear film 92 arecollected and are observable. Thereafter, the patient's 184 tear film 92is illuminated by the illuminator 196 (block 272).

The imaging device 194 is then controlled and focused on the lipid layer96 to collect specularly reflected light from an area or ROI 134 on atear film 92 as a result of illuminating the tear film 92 with theilluminator 196 in a first image (block 274, FIG. 20). An example of thefirst image by the illuminator 196 is provided in FIG. 9. As illustratedtherein, a first image 130 of a patient's eye 132 is shown that has beenilluminated with the illuminator 196. The illuminator 196 and theimaging device 194 may be controlled to illuminate an area or ROI 134 ona tear film 136 that does not include a pupil 138 of the eye 132 so asto reduce reflex tearing. Reflex tearing will temporarily lead tothicker aqueous and lipid layers, thus temporarily altering theinterference signals of specularly reflected light from the tear film136. As shown in FIG. 9, when the imaging device 194 is focused on ananterior surface 142 of the lipid layer 144 of the tear film 136,interference interactions 140 of the interference signal of thespecularly reflected light from the tear film 136 as a result ofillumination by the illuminator 196 are captured in the area or ROI 134in the first image 130. The interference interactions 140 appear to ahuman observer as colored patterns as a result of the wavelengthspresent in the interference of the specularly reflected light from thetear film 136.

However, even though the background signal is reduced by the firstpolarizer 52(2), a portion of the background signal is also captured inthe first image 130. The background signal is added to the specularlyreflected light in the area or ROI 134 and included outside the area orROI 134 as well. Background signal is light that is not specularlyreflected from the tear film 136 and thus contains no interferenceinformation. Background signal can include stray and ambient lightentering into the imaging device 194, scattered light from the patient's184 face, eyelids, and/or eye 132 structures outside and beneath thetear film 136 as a result of stray light, ambient light and diffuseillumination by the illuminator 196, and images of structures beneaththe tear film 136. For example, the first image 130 includes the iris ofthe eye 132 beneath the tear film 136. Background signal adds a bias(i.e., offset) error to the captured interference of specularlyreflected light from the tear film 136 thereby reducing its signalstrength and contrast. Further, if the background signal has a color huedifferent from the light of the light source, a color shift can alsooccur due to the interference of specularly reflected light from thetear film 136 in the first image 130. The imaging device 194 produces afirst output signal that represents the light rays captured in the firstimage 130. Because the first image 130 contains light rays fromspecularly reflected light as well as the background signal, the firstoutput signal produced by the imaging device 194 from the first image130 will contain an interference signal representing the capturedinterference of the specularly reflected light from the tear film 136with a bias (i.e., offset) error caused by the background signal. As aresult, the first output signal analyzed to measure TFLT may containerror as a result of the background signal bias (i.e., offset) error.

Thus, in this embodiment, the first output signal generated by theimaging device 194 as a result of the first image 130 is processed tosubtract or substantially subtract the background signal from theinterference signal to reduce error before being analyzed to measureTFLT. This is also referred to as “background subtraction.” Backgroundsubtraction is the process of removing unwanted reflections from images.In this regard, the imaging device 194 is controlled to capture a secondimage 146 of the tear film 136. However, before the second image 146 iscaptured, the polarizer wheel 260 is rotated such that the secondpolarizer 56(2) is disposed in the imaging path of the imaging device194 in a second orientation that is polarized 90° relative to the firstpolarization orientation (block 276 in FIG. 20). In this way, the secondimage 146 will contain mostly background signal and when the secondimage 146 is subtracted from the first image 130 the capturedinterference of specularly reflected light from the tear film 136 willnot be reduced or at least not significantly reduced.

The second image 146 should be captured using the same imaging device194 settings and focal point as when capturing the first image 130 sothat the first image 130 and second image 146 form corresponding imagepairs captured within a short time of each other. The imaging device 194produces a second output signal containing background signal present inthe first image 130 (block 278 in FIG. 20). To eliminate or reduce thisbackground signal from the first output signal, the second output signalis subtracted from the first output signal to produce a resulting signal(block 280 in FIG. 20). The image representing the resulting signal inthis example is illustrated in FIG. 11 as resulting image 148 (block 282in FIG. 20). Thus, in this example, background subtraction involves twoimages 130, 146 to provide a frame pair where the two images 130, 146are subtracted from each other, whereby specular reflection from thetear film 136 is retained, and while diffuse reflections from the irisand other areas are removed in whole or part.

OSI Device Employing Two Imaging Devices/Non-Polarizing Beam Splitter

FIG. 21A is a side view and FIG. 21B is a top view of the OSI device 170with a dual imaging device configuration. The first imaging device 194is configured to capture the at least one first image, and a secondimaging device 194(2) is configured to capture the at least one secondimage. A non-polarizing beam splitter 286 is configured to direct afirst portion of the specularly reflected light including the backgroundsignal to the first imaging device 194 while simultaneously directing asecond portion of the specularly reflected light including thebackground signal to the second imaging device 194(2).

A first polarizer 52(3) is disposed in an imaging path of the firstimaging device 194. The first polarizer 52(3) has a polarization axisthat is parallel or substantially parallel to the polarization plane ofthe specularly reflected light from the ROI 134 of the ocular tear filmto pass or substantially pass the specularly reflected light to thefirst imaging device 194 while reducing the background signal. A secondpolarizer 56(3) is disposed in an imaging path of a second imagingdevice 194(2). The second polarizer 56(3) has a polarization axis thatis perpendicular or substantially perpendicular to the polarizationplane of the specularly reflected light from the ROI 134 of the oculartear film 92 to reduce or eliminate the specularly reflected light whilepassing a portion of background signal to the second imaging device194(2). In this exemplary case, the second imaging device 194(2) is avideo camera 198(1), which is the same type and model as the videocamera 198.

FIG. 22 is a flowchart of an exemplary process for the dual cameraconfiguration of FIGS. 21A-21B, FIGS. 23A-23B, and FIGS. 24A-24B forobtaining one or more interference signals from images of a tear film 92representing specularly reflected light from the tear film 92 withbackground signal subtracted or substantially subtracted. Against thebackdrop of the OSI device 170 in FIGS. 21A-21B, FIGS. 23A-23B, andFIGS. 24A-24B, FIG. 22 illustrates a flowchart discussing how the OSIdevice 170 can be used to obtain interference interactions of specularlyreflected light from the tear film 92, which can be used to measureTFLT. Interference interactions of specularly reflected light from thetear film 92 are first obtained and discussed before measurement of TFLTis discussed. In this embodiment as illustrated in FIG. 22, the processstarts by adjusting the patient 184 with regard to an illuminator 196and an imaging device 194 (block 290). The illuminator 196 is controlledto illuminate the patient's 184 tear film 92. The imaging device 194 iscontrolled to be focused on the anterior surface 98 of the lipid layer96 such that the interference interactions of specularly reflected lightfrom the tear film 92 are collected and are observable. Thereafter, thepatient's 184 tear film 92 is illuminated by the illuminator 196 (block292).

The imaging device 194 is then controlled and focused on the lipid layer96 to collect specularly reflected light from an area or ROI 134 on atear film 92 as a result of illuminating the tear film 92 with theilluminator 196 in a first image (block 294, FIG. 22). An example of thefirst image by the illuminator 196 is provided in FIG. 9. As illustratedtherein, a first image 130 of a patient's eye 132 is shown that has beenilluminated with the illuminator 196. The illuminator 196 and theimaging device 194 may be controlled to illuminate an area or ROI 134 ona tear film 136 that does not include a pupil 138 of the eye 132 so asto reduce reflex tearing. Reflex tearing will temporarily lead tothicker aqueous and lipid layers, thus temporarily altering theinterference signals of specularly reflected light from the tear film136. As shown in FIG. 9, when the imaging device 194 is focused on ananterior surface 142 of the lipid layer 144 of the tear film 136,interference interactions 140 of the interference signal of thespecularly reflected light from the tear film 136 as a result ofillumination by the illuminator 196 are captured in the area or ROI 134in the first image 130. The interference interactions 140 appear to ahuman observer as colored patterns as a result of the wavelengthspresent in the interference of the specularly reflected light from thetear film 136.

However, even though the background signal is reduced by the firstpolarizer 52(3), a portion of the background signal is also captured inthe first image 130. The background signal is added to the specularlyreflected light in the area or ROI 134 and included outside the area orROI 134 as well. Background signal is light that is not specularlyreflected from the tear film 136 and thus contains no interferenceinformation. Background signal can include stray and ambient lightentering into the imaging device 194, scattered light from the patient's184 face, eyelids, and/or eye 132 structures outside and beneath thetear film 136 as a result of stray light, ambient light and diffuseillumination by the illuminator 196, and images of structures beneaththe tear film 136. For example, the first image 130 includes the iris ofthe eye 132 beneath the tear film 136. Background signal adds a bias(i.e., offset) error to the captured interference of specularlyreflected light from the tear film 136 thereby reducing its signalstrength and contrast. Further, if the background signal has a color huedifferent from the light of the light source, a color shift can alsooccur due to the interference of specularly reflected light from thetear film 136 in the first image 130. The imaging device 194 produces afirst output signal that represents the light rays captured in the firstimage 130. Because the first image 130 contains light rays fromspecularly reflected light as well as the background signal, the firstoutput signal produced by the imaging device 194 from the first image130 will contain an interference signal representing the capturedinterference of the specularly reflected light from the tear film 136with a bias (i.e., offset) error caused by the background signal. As aresult, the first output signal analyzed to measure TFLT may containerror as a result of the background signal bias (i.e., offset) error.

Thus, in this embodiment, the first output signal generated by theimaging device 194 as a result of the first image 130 is processed tosubtract or substantially subtract the background signal from theinterference signal to reduce error before being analyzed to measureTFLT. This is also referred to as “background subtraction.” Backgroundsubtraction is the process of removing unwanted reflections from images.In this regard, the second imaging device 194(2) is controlledsimultaneously with the first imaging device 194 to capture a secondimage 146 of the tear film 136. In this way, the second image 146 willcontain mostly background signal and when the second image 146 issubtracted from the first image 130 the captured interference ofspecularly reflected light from the tear film 136 will not be reduced orat least not significantly reduced.

The second imaging device 194(2) produces a second output signalcontaining background signal present in the first image 130 (block 294in FIG. 22). To eliminate or reduce this background signal from thefirst output signal, the second output signal is subtracted from thefirst output signal to produce a resulting signal (block 296 in FIG.22). The image(s) representing the resulting signal(s) containinginterference signal(s) of specularly reflected light from the tear filmis produced (block 298 in FIG. 22). In this example the resulting image148 is illustrated in FIG. 11. Thus, in this example, backgroundsubtraction involves two images 130, 146 to provide a frame pair wherethe two images 130, 146 are subtracted from each other, whereby specularreflection from the tear film 136 is retained, and while diffusereflections from the iris and other areas are removed in whole or part.

OSI Device Employing Two Imaging Devices/Polarizing Beam Splitter

FIG. 23A is a side view and FIG. 23B is a top view of the OSI devicewith yet another dual imaging device configuration. In this embodiment,the first polarizer 52(3) and the second polarizer 56(3) of FIGS. 21Aand 21B are replaced by a polarizing beam splitter 300. However, theoperation of the embodiment is operated as described by the flowchart ofFIG. 22.

In this embodiment, specularly reflected light from the tear film 136passes largely unimpeded through the polarizing beam splitter 300 andonward to the first imaging device 194. A portion of background signalis also transmitted through the polarizing beam splitter 300 to thefirst imaging device 194. In contrast, specularly reflected light fromthe tear film 136 is practically blocked from reflecting to the secondimaging device 194(1), while a portion of the background signal isdirected to the second imaging device 194(1).

OSI Device Employing Illuminator Polarizer

FIG. 24A is a side view and FIG. 24B is a top view of the OSI device ofFIG. 23A and FIG. 23B that further includes a polarizer 302. Thepolarizer 302 is disposed in the illumination path of the illuminator196 to polarize multi-wavelength light emitted from the illuminator 196that is illuminating the ROI 134 of the ocular tear film 136. Thepolarizer 302 helps further isolate by reducing the amount ofunpolarized light that makes up the background signal. The polarizer 302can be either a linear polarizer or a circular polarizer.

System Level

Now that the imaging and illumination functions of the OSI device 170have been described, FIG. 25A illustrates a system level diagramillustrating more detail regarding the control system and other internalcomponents of the OSI device 170 provided inside the housing 172according to one embodiment to capture images of a patient's tear filmand process those images. As illustrated therein, a control system 340is provided that provides the overall control of the OSI device 170. Thecontrol system 340 may be provided by any microprocessor-based orcomputer system. The control system 340 illustrated in FIG. 25A isprovided in a system-level diagram and does not necessarily imply aspecific hardware organization and/or structure. As illustrated therein,the control system 340 contains several systems. A polarization settingssystem 341 may be provided that accepts input from a clinician user. Thepolarization settings may include but are not limited to synchronizationvalues that adjust video camera and polarizer synchronization. A camerasettings system 342 may be provided that accepts camera settings from aclinician user. Exemplary camera settings 344 are illustrated, but maybe any type according to the type and model of camera provided in theOSI device 170 as is well understood by one of ordinary skill in theart.

The camera settings 344 may be provided to (The Imaging Source) cameradrivers 346, which may then be loaded into the video camera 198 uponinitialization of the OSI device 170 for controlling the settings of thevideo camera 198. The settings and drivers may be provided to a buffer348 located inside the video camera 198 to store the settings forcontrolling a CCD 350 for capturing ocular image information from a lens352. Ocular images captured by the lens 352 and the CCD 350 are providedto a de-Bayering function 354 which contains an algorithm forpost-processing of raw data from the CCD 350 as is well known. Theocular images are then provided to a video acquisition system 356 in thecontrol system 340 and stored in memory, such as random access memory(s)(RAM(s)) 358. The stored ocular images or signal representations canthen be provided to a pre-processing system 360 and a post-processingsystem 362 to manipulate the ocular images to obtain the interferenceinteractions of the specularly reflected light from the tear film andanalyze the information to determine characteristics of the tear film.Pre-processing settings 364 and post-processing settings 366 can beprovided to the pre-processing system 360 and post-processing system362, respectively, to control these functions. The pre-processingsettings 364 and the post-processing settings 366 will be described inmore detail below. The post-processed ocular images and information mayalso be stored in mass storage, such as disk memory 368, for laterretrieval and viewing on the display 174.

The control system 340 may also contain a visualization system 370 thatprovides the ocular images to the display 174 to be displayed inhuman-perceptible form on the display 174. Before being displayed, theocular images may have to be pre-processed in a pre-processing videofunction 372. For example, if the ocular images are provided by a linearcamera, non-linearity (i.e. gamma correction) may have to be added inorder for the ocular images to be properly displayed on the display 174.Further, contrast and saturation display settings 374, which may becontrolled via the display 174 or a device communicating to the display174, may be provided by a clinician user to control the visualization ofocular images displayed on the display 174. The display 174 is alsoadapted to display analysis result information 376 regarding thepatient's tear film, as will be described in more detail below. Thecontrol system 340 may also contain a user interface system 378 thatdrives a graphical user interface (GUI) utility 380 on the display 174to receive user input 382. The user input 382 can include any of thesettings for the OSI device 170, including the camera settings 344, thepre-processing settings 364, the post-processing settings 366, thedisplay settings 374, the visualization system 370 enablement, and videoacquisition system 356 enablement, labeled 1-6. The GUI utility 380 mayonly be accessible by authorized personnel and used for calibration orsettings that would normally not be changed during normal operation ofthe OSI device 170 once configured and calibrated.

Overall Process Flow

FIG. 25B illustrates an exemplary overall flow process performed by theOSI device 170 for capturing tear film images from a patent and analysisfor TFLT measurement. As illustrated in FIG. 25B, the first video camera198 and an optional second video camera 198(1) are connected via a USBport(s) 383 to the control system 340 (see FIG. 25A) for control of thefirst video camera 198 and the second video camera 198(1) and fortransferring images of a patient's tear film taken by the video camera198 back to the control system 340. The control system 340 includes acompatible camera driver 346 to provide a transfer interface between thecontrol system 340 and the video camera 198. Prior to tear film imagecapture, the configuration or camera settings 344 are loaded into thevideo camera 198 over the USB port 383 to prepare the video camera 198for tear film image capture (block 385). Further, an audio videointerleaved (AVI) container is created by the control system 340 tostore video of tear film images to be captured by the video camera 198(block 386). At this point, the video camera 198 and control system 340are ready to capture images of a patient's tear film. The control system340 waits for a user command to initiate capture of a patient's tearfilm (blocks 387, 388).

Once image capture is initiated (block 388), the control system enablesimage capture to the AVI container previously setup (block 386) forstorage of images captured by the video camera 198 (block 389). Thecontrol system 340 controls the video camera 198 to capture images ofthe patient's tear film (block 389) until timeout or the user terminatesimage capture (block 390) and image capture halts or ends (block 391).Images captured by the video camera 198 and provided to the controlsystem 340 over the USB port 383 are stored by the control system 340 inRAM(S) 358.

The captured images of the patient's ocular tear film can subsequentlybe processed and analyzed to perform TFLT measurement, as described inmore detail below and throughout the remainder of this disclosure. Theprocess in this embodiment involves processing tear film image pairs toperform background subtraction, as previously discussed. The processingcan include simply displaying the patient's tear film or performing TFLTmeasurement (block 393). If the display option is selected to allow atechnician to visually view the patient's tear film, display processingis performed (block 394) which can be the display processing 370described in more detail below with regard to FIG. 34. For example, thecontrol system 340 can provide a combination of images of the patient'stear film that show the entire region of interest of the tear film onthe display 174. The displayed image may include the background signalor may have the background signal subtracted. If TFLT measurement isdesired, the control system 340 performs pre-processing of the tear filmimages for TFLT measurement (block 395), which can be the pre-processingsystem 360 described in more detail below with regard to FIG. 26. Thecontrol system 340 also performs post-processing of the tear film imagesfor TFLT measurement (block 396), which can be the post-processingsystem 362 described in more detail below with regard to FIG. 36.

Pre-Processing

FIG. 26 illustrates an exemplary pre-processing system 360 forpre-processing ocular tear film images captured by the OSI device 170for eventual analysis and TFLT measurement. In this system, the videocamera 198 has already taken the first and second images of a patient'socular tear film, as previously illustrated in FIGS. 9 and 10, andprovided the images to the video acquisition system 356. The frames ofthe first and second images were then loaded into RAM(s) 358 by thevideo acquisition system 356. Thereafter, as illustrated in FIG. 26, thecontrol system 340 commands the pre-processing system 360 to pre-processthe first and second images. An exemplary GUI utility 380 is illustratedin FIG. 27 that may be employed by the control system 340 to allow aclinician to operate the OSI device 170 and control pre-processingsettings 364 and post-processing settings 366, which will be describedlater in this application. In this regard, the pre-processing system 360loads the first and second image frames of the ocular tear film fromRAM(s) 358 (block 400). The exemplary GUI utility 380 in FIG. 27 allowsfor a stored image file of previously stored video sequence of first andsecond image frames captured by the video camera 198 by entering a filename in the file name field 451. A browse button 452 also allowssearches of the memory for different video files, which can either bebuffered by selecting a buffered box 454 or loaded for pre-processing byselecting the load button 456.

If the loaded first and second image frames of the tear film arebuffered, they can be played using display selection buttons 458, whichwill in turn display the images on the display 174. The images can beplayed on the display 174 in a looping fashion, if desired, by selectingthe loop video selection box 460. A show subtracted video selection box470 in the GUI utility 380 allows a clinician to show the resulting,subtracted video images of the tear film on the display 174representative of the resulting signal comprised of the second outputsignal combined or subtracted from the first output signal, or viceversa. Also, by loading the first and second image frames, thepreviously described subtraction technique can be used to removebackground image from the interference signal representing interferenceof the specularly reflected light from the tear film, as previouslydescribed above and illustrated in FIG. 11 as an example. The firstimage is subtracted from the second image to subtract or remove thebackground signal in the portions producing specularly reflected lightin the second image, and vice versa, and then combined to produce aninterference interaction of the specularly reflected light of the entirearea or region of interest of the tear film, as previously illustratedin FIG. 11 (block 402 in FIG. 26). For example, this processing could beperformed using the Matlab® function “cvAbsDiff.”

The subtracted image containing the specularly reflected light from thetear film can also be overlaid on top of the original image capture ofthe tear film to display an image of the entire eye and the subtractedimage in the display 174 by selecting the show overlaid original videoselection box 462 in the GUI utility 380 of FIG. 27. An example of anoverlaid original video to the subtracted image of specularly reflectedlight from the tear film is illustrated in the image 520 of FIG. 28.This overlay is provided so that flashing images of specularly reflectedlight from the tear film are not displayed, which may be unpleasant tovisualize. The image 520 of the tear film illustrated in FIG. 28 wasobtained with a DBK 21AU04 Bayer VGA (640×480) video camera having aPentax VS-LD25 Daitron 25-mm fixed focal length lens with maximumaperture at a working distance of 120 mm and having the followingsettings, as an example:

-   -   Gamma=100 (to provide linearity with exposure value)    -   Exposure= 1/16 second    -   Frame rate=60 fps    -   Data Format=BY8    -   Video Format=-uncompressed, RGB 24-bit AVI    -   Hue=180 (neutral, no manipulation)    -   Saturation=128(neutral, no manipulation)    -   Brightness=0 (neutral, no manipulation)    -   Gain=260 (minimum available setting in this camera driver)    -   White balance=B=78; R=20.        Thresholding

Any number of optional pre-processing steps and functions can next beperformed on the resulting combined tear film image(s), which will nowbe described. For example, an optional threshold pre-processing functionmay be applied to the resulting image or each image in a video of imagesof the tear film (e.g., FIG. 11) to eliminate pixels that have asubtraction difference signal below a threshold level (block 404 in FIG.26). Image threshold provides a black and white mask (on/off) that isapplied to the tear film image being processed to assist in removingresidual information that may not be significant enough to be analyzedand/or may contribute to inaccuracies in analysis of the tear film. Thethreshold value used may be provided as part of a threshold valuesetting provided by a clinician as part of the pre-processing settings364, as illustrated in the system diagram of FIG. 25A. For example, theGUI utility 380 in FIG. 27 includes a compute threshold selection box472 that may be selected to perform thresholding, where the thresholdbrightness level can be selected via the threshold value slide 474. Thecombined tear film image of FIG. 12 is copied and converted tograyscale. The grayscale image has a threshold applied according to thethreshold setting to obtain a binary (black/white) image that will beused to mask the combined tear film image of FIG. 11. After the mask isapplied to the combined tear film image of FIG. 11, the new combinedtear film image is stored in RAM(s) 358. The areas of the tear filmimage that do not meet the threshold brightness level are converted toblack as a result of the threshold mask.

FIGS. 29A and 29B illustrate examples of threshold masks for thecombined tear film provided in FIG. 11. FIG. 29A illustrates a thresholdmask 522 for a threshold setting of 70 counts out of a full scale levelof 255 counts. FIG. 29B illustrates a threshold mask 524 for a thresholdsetting of 50. Note that the threshold mask 522 in FIG. 29A containsless portions of the combined tear film image, because the thresholdsetting is higher than for the threshold mask 524 of FIG. 29B. When thethreshold mask according to a threshold setting of 70 is applied to theexemplary combined tear film image of FIG. 11, the resulting tear filmimage is illustrated FIG. 30. Much of the residual subtracted backgroundimage that surrounds the area or region of interest has been maskedaway.

Erode and Dilate

Another optional pre-processing function that may be applied to theresulting image or each image in a video of images of the tear film tocorrect anomalies in the combined tear film image(s) is the erode anddilate functions (block 406 in FIG. 26). The erode function generallyremoves small anomaly artifacts by subtracting objects with a radiussmaller than an erode setting (which is typically in number of pixels)removing perimeter pixels where interference information may not be asdistinct or accurate. The erode function may be selected by a clinicianin the GUI utility 380 (see FIG. 27) by selecting the erode selectionbox 476. If selected, the number of pixels for erode can be provided inan erode pixels text box 478. Dilating generally connects areas that areseparated by spaces smaller than a minimum dilate size setting by addingpixels of the eroded pixel data values to the perimeter of each imageobject remaining after the erode function is applied. The dilatefunction may be selected by a clinician in the GUI utility 380 (see FIG.27) by providing the number of pixels for dilating in a dilate pixelstext box 480. Erode and dilate can be used to remove small regionanomalies in the resulting tear film image prior to analyzing theinterference interactions to reduce or avoid inaccuracies. Theinaccuracies may include those caused by bad pixels of the video camera198 or from dust that may get onto a scanned image, or more commonly,spurious specular reflections such as: tear film meniscus at thejuncture of the eyelids, glossy eyelash glints, wet skin tissue, etc.FIG. 31 illustrates the resulting tear film image of FIG. 30 after erodeand dilate functions have been applied and the resulting tear film imageis stored in RAM(s) 358. As illustrated therein, pixels previouslyincluded in the tear film image that was not in the tear film area orregion of interest are removed. This prevents data in the image outsidethe area or region of interest from affecting the analysis of theresulting tear film image(s).

Removing Blinks/Other Anomalies

Another optional pre-processing function that may be applied to theresulting image or each image in a video of images of the tear film tocorrect anomalies in the resulting tear film image is to remove framesfrom the resulting tear film image that include patient blinks orsignificant eye movements (block 408 in FIG. 26). As illustrated in FIG.26, blink detection is shown as being performed after a threshold anderode and dilate functions are performed on the tear film image or videoof images. Alternatively, the blink detection could be performedimmediately after background subtraction, such that if a blink isdetected in a given frame or frames, the image in such frame or framescan be discarded and not pre-processed. Not pre-processing images whereblinks are detected may increase the overall speed of pre-processing.The remove blinks or movement pre-processing may be selectable. Forexample, the GUI utility 380 in FIG. 27 includes a remove blinksselection box 484 to allow a user to control whether blinks and/or eyemovements are removed from a resulting image or frames of the patient'stear film prior to analysis. Blinking of the eyelids covers the oculartear film, and thus does not produce interference signals representingspecularly reflected light from the tear film. If frames containingwhole or partial blinks obscuring the area or region of interest in thepatient's tear film are not removed, it would introduce errors in theanalysis of the interference signals to determine characteristics of theTFLT of the patient's ocular tear film. Further, frames or data withsignificant eye movement between sequential images or frames can beremoved during the detect blink pre-processing function. Large eyemovements could cause inaccuracy in analysis of a patient's tear filmwhen employing subtraction techniques to remove background signal,because subtraction involves subtracting frame-pairs in an image thatclosely match spatially. Thus, if there is significant eye movementbetween first and second images that are to be subtracted, frame pairsmay not be closely matched spatially thus inaccurately removingbackground signal, and possibly removing a portion of the interferenceimage of specularly reflected light from the tear film.

Different techniques can be used to determine blinks in an ocular tearfilm image and remove the frames as a result. For example, in oneembodiment, the control system 340 directs the pre-processing system 360to review the stored frames of the resulting images of the tear film tomonitor for the presence of an eye pupil using pattern recognition. AHough Circle Transform may be used to detect the presence of the eyepupil in a given image or frame. If the eye pupil is not detected, it isassembled such that the image or frame contains an eye blink and thusshould be removed or ignored during pre-processing from the resultingimage or video of images of the tear film. The resulting image or videoof images can be stored in RAM(s) 358 for subsequent processing and/oranalyzation.

In another embodiment, blinks and significant eye movements are detectedusing a histogram sum of the intensity of pixels in a resultingsubtracted image or frame of a first and second image of the tear film.An example of such a histogram 526 is illustrated in FIG. 32. Theresulting or subtracted image can be converted to grayscale (i.e., 255levels) and a histogram generated with the gray levels of the pixels. Inthe histogram 526 of FIG. 32, the x-axis contains gray level ranges, andthe number of pixels falling within each gray level is contained in they-axis. The total of all the histogram 526 bins are summed. In the caseof two identical frames that are subtracted, the histogram sum would bezero. However, even without an eye blink or significant eye movement,two sequentially captured frames of the patient's eye and theinterference signals representing the specularly reflected light fromthe tear film are not identical. However, frame pairs with littlemovement will have a low histogram sum, while frame pairs with greatermovement will yield a larger histogram sum. If the histogram sum isbeyond a pre-determined threshold, an eye blink or large eye movementcan be assumed and the image or frame removed. For example, the GUIutility 380 illustrated in FIG. 27 includes a histogram sum slide bar486 that allows a user to set the threshold histogram sum. The thresholdhistogram sum for determining whether a blink or large eye movementshould be assumed and thus the image removes from analysis of thepatient's tear film can be determined experimentally, or adaptively overthe course of a frame playback, assuming that blinks occur at regularintervals.

An advantage of a histogram sum of intensity method to detect eye blinksor significant eye movements is that the calculations are highlyoptimized as opposed to pixel-by-pixel analysis, thus assisting withreal-time processing capability. Further, there is no need to understandthe image structure of the patient's eye, such as the pupil or the irisdetails. Further, the method can detect both blinks and eye movements.

Another alternate technique to detect blinks in the tear film image orvideo of images for possible removal is to calculate a simple averagegray level in an image or video of images. Because the subtracted,resulting images of the tear film subtract background signal, and havebeen processed using a threshold mask, and erode and dilate functionsperformed in this example, the resulting images will have a loweraverage gray level due to black areas present than if a blink ispresent. A blink contains skin color, which will increase the averagegray level of an image containing a blink. A threshold average graylevel setting can be provided. If the average gray level of a particularframe is below the threshold, the frame is ignored from further analysisor removed from the resulting video of frames of the tear film.

Another alternate technique to detect blinks in an image or video ofimages for removal is to calculate the average number of pixels in agiven frame that have a gray level value below a threshold gray levelvalue. If the percentage of pixels in a given frame is below a definedthreshold percentage, this can be an indication that a blink hasoccurred in the frame, or that the frame is otherwise unworthy ofconsideration when analyzing the tear film. Alternatively, a spatialfrequency calculation can be performed on a frame to determine theamount of fine detail in a given frame. If the detail present is below athreshold detail level, this may be an indication of a blink or otherobscurity of the tear film, since skin from the eyelid coming down andbeing captured in a frame will have less detail than the subtractedimage of the tear film. A histogram can be used to record any of theabove-referenced calculations to use in analyzing whether a given frameshould be removed from the final pre-processed resulting image or imagesof the tear film for analyzation.

ICC Profiling

Pre-processing of the resulting tear film image(s) may also optionallyinclude applying an International Colour Consortium (ICC) profile to thepre-processed interference images of the tear film (block 410, FIG. 26).FIG. 33 illustrates an optional process of loading an ICC profile intoan ICC profile 531 in the control system 340 (block 530). In thisregard, the GUI utility 380 illustrated in FIG. 27 also includes anapply ICC box 492 that can be selected by a clinician to load the ICCprofile 531. The ICC profile 531 may be stored in memory in the controlsystem 340, including in RAM(s) 358. In this manner, the GUI utility 380in FIG. 27 also allows for a particular ICC profile 531 to be selectedfor application in the ICC profile file text box 494. The ICC profile531 can be used to adjust color reproduction from scanned images fromcameras or other devices into a standard red-green-blue (RGB) colorspace (among other selectable standard color spaces) defined by the ICCand based on a measurement system defined internationally by theCommission Internationale de l'Eclairage (CIE). Adjusting thepre-processed resulting tear film interference images corrects forvariations in the camera color response and the light source spectrumand allows the images to be compatibly compared with a tear film layerinterference model to measure the thickness of a TFLT, as will bedescribed later in this application. The tear film layers represented inthe tear film layer interference model can be LLTs, ALTs, or both, aswill be described in more detail below.

In this regard, the ICC profile 531 may have been previously loaded tothe OSI device 170 before imaging of a patient's tear film and alsoapplied to a tear film layer interference model when loaded into the OSIdevice 170 independent of imaging operations and flow. As will bediscussed in more detail below, a tear film layer interference model inthe form of a TFLT palette 533 containing color values representinginterference interactions from specularly reflected light from a tearfilm for various LLTs and ALTs can also be loaded into the OSI device170 (block 532 in FIG. 36). The TFLT palette 533 contains a series ofcolor values that are assigned LLTs and/or ALTs based on a theoreticaltear film layer interference model to be compared against the colorvalue representations of interference interactions in the resultingimage(s) of the patient's tear film. When applying the optional ICCprofile 531 to the TFLT palette 533 (block 534 in FIG. 33), the colorvalues in both the tear film layer interference model and the colorvalues representing interference interactions in the resulting image ofthe tear film are adjusted for a more accurate comparison between thetwo to measure LLT and/or ALT.

Brightness

Also as an optional pre-processing step, brightness and red-green-blue(RGB) subtract functions may be applied to the resulting interferencesignals of the patient's tear film before post-processing for analysisand measuring TFLT is performed (blocks 412 and 414 in FIG. 26). Thebrightness may be adjusted pixel-by-pixel by selecting the adjustbrightness selection box 504 according to a corresponding brightnesslevel value provided in a brightness value box 506, as illustrated inthe GUI utility 380 of FIG. 27. When the brightness value box 506 isselected, the brightness of each palette value of the TFLT palette 533is also adjusted accordingly.

RGB Subtraction (Normalization)

The RGB subtract function subtracts a DC offset from the interferencesignal in the resulting image(s) of the tear film representing theinterference interactions in the interference signal. An RGB subtractsetting may be provided from the pre-processing settings 364 to apply tothe interference signal in the resulting image of the tear film tonormalize against. As an example, the GUI utility 380 in FIG. 27 allowsan RGB offset to be supplied by a clinician or other technician for usein the RGB subtract function. As illustrated therein, the subtract RGBfunction can be activated by selecting the RGB subtract selection box496. If selected, the individual RGB offsets can be provided in offsetvalue input boxes 498. After pre-processing is performed, if any, on theresulting image, the resulting image can be provided to apost-processing system to measure TLFT (block 416), as discussed laterbelow in this application.

Displaying Images

The resulting images of the tear film may also be displayed on thedisplay 174 of the OSI device 170 for human diagnosis of the patient'socular tear film. The OSI device 170 is configured so that a cliniciancan display and see the raw captured image of the patient's eye 192 bythe video camera 198, the resulting images of the tear film beforepre-processing, or the resulting images of the tear film afterpre-processing. Displaying images of the tear film on the display 174may entail different settings and steps. For example, if the videocamera 198 provides linear images of the patient's tear film, the linearimages must be converted into a non-linear format to be properlydisplayed on the display 174. In this regard, a process that isperformed by the visualization system 370 according to one embodiment isillustrated in FIG. 34.

As illustrated in FIG. 34, the video camera 198 has already taken thefirst and second images of a patient's ocular tear film as previouslyillustrated in FIGS. 9 and 10, and provided the images to the videoacquisition system 356. The frames of the first and second images werethen loaded into RAM(s) 358 by the video acquisition system 356.Thereafter, as illustrated in FIG. 34, the control system 340 commandsthe visualization system 370 to process the first and second images toprepare them for being displayed on the display 174, 538. In thisregard, the visualization system 370 loads the first and second imageframes of the ocular tear film from RAM(s) 358 (block 535). Thepreviously described subtraction technique is used to remove backgroundsignal from the interference interactions of the specularly reflectedlight from the tear film, as previously described above and illustratedin FIG. 11. The first image(s) is subtracted from the second image(s) toremove background signal in the illuminated portions of the firstimage(s), and vice versa, and the subtracted images are then combined toproduce an interference interaction of the specularly reflected light ofthe entire area or region of interest of the tear film, as previouslydiscussed and illustrated in FIG. 11 (block 536 in FIG. 34).

Again, for example, this processing could be performed using the Matlab®function “cvAbsDiff.” Before being displayed, the contrast andsaturation levels for the resulting images can be adjusted according tocontrast and saturation settings provided by a clinician via the userinterface system 378 and/or programmed into the visualization system 370(block 537). For example, the GUI utility 380 in FIG. 27 provides anapply contrast button 464 and a contrast setting slide 466 to allow theclinician to set the contrast setting in the display settings 374 fordisplay of images on the display 174. The GUI utility 380 also providesan apply saturation button 468 and a saturation setting slide 469 toallow a clinician to set the saturation setting in the display settings374 for the display of images on the display 174. The images can then beprovided by the visualization system 370 to the display 174 fordisplaying (block 538 in FIG. 34). Also, any of the resulting imagesafter pre-processing steps in the pre-processing system 360 can beprovided to the display 174 for processing.

FIGS. 35A-35C illustrate examples of different tear film images that aredisplayed on the display 174 of the OSI device 170. FIG. 35A illustratesa first image 539 of the patient's tear film showing the patterncaptured by the video camera 198. This image is the same image asillustrated in FIG. 9 and previously described above, but processed froma linear output from the video camera 198 to be properly displayed onthe display 174. FIG. 35B illustrates a second image 540 of thepatient's tear film illustrated in FIG. 10 and previously describedabove. FIG. 35C illustrates a resulting “overlaid” image 341 of thefirst and second images 539, 540 of the patient's tear film and toprovide interference interactions of the specularly reflected light fromthe tear film over the entire area or region of interest. This is thesame image as illustrated in FIG. 11 and previously described above.

In this example, the original number of frames of the patient's tearfilm captured can be reduced if frames in the subtracted image framescapture blinks or erratic movements, and these frames are eliminated inpre-processing, a further reduction in frames will occur duringpre-processing from the number of images raw captured in images of thepatient's tear film. Although these frames are eliminated from beingfurther processed, they can be retained for visualization rendering arealistic and natural video playback. Further, by applying athresholding function and erode and dilating functions, the number ofnon-black pixels which contain TLFT interference information issubstantially reduced as well. Thus, the amount of pixel informationthat is processed by the post-processing system 362 is reduced, and maybe on the order of 70% less information to process than the raw imagecapture information, thereby pre-filtering for the desired interferenceROI and reducing or elimination potentially erroneous information aswell as allowing for faster analysis due to the reduction ininformation.

At this point, the resulting images of the tear film have beenpre-processed by the pre-processing system 360 according to whateverpre-processing settings 364 and pre-processing steps have been selectedor implemented by the control system 340. The resulting images of thetear film are ready to be processed for analyzing and determining TFLT.In this example, this is performed by the post-processing system 362 inFIG. 25A and is based on the post-processing settings 366 alsoillustrated therein. An embodiment of the post-processing performed bythe post-processing system 362 is illustrated in the flowchart of FIG.36.

Tear Film Interference Models

As illustrated in FIG. 36, pre-processed images 543 of the resultingimages of the tear film are retrieved from RAM(s) 358 where they werepreviously stored by the pre-processing system 360. Before discussingthe particular embodiment of the post-processing system 362 in FIG. 36,in general, to measure TFLT, the RGB color values of the pixels in theresulting images of the tear film are compared against color valuesstored in a tear film interference model that has been previously loadedinto the OSI device 170 (see FIG. 33. The tear film interference modelmay be stored as a TFLT palette 533 containing RGB values representinginterference colors for given LLTs and/or ALTs. The TFLT palettecontains interference color values that represent TFLTs based on atheoretical tear film interference model in this embodiment. Dependingon the TFLT palette provided, the interference color values representedtherein may represent LLTs, ALTs, or both. An estimation of TFLT foreach ROI pixel is based on this comparison. This estimate of TFLT isthen provided to the clinician via the display 174 and/or recorded inmemory to assist in diagnosing DES.

Before discussing embodiments of how the TFLTs are estimated from thepre-processed resulting image colored interference interactionsresulting from specularly reflected light from the tear film, tear filminterference modeling is first discussed. Tear film interferencemodeling can be used to determine an interference color value for agiven TFLT to measure TFLT, which can include both LLT and/or ALT.

Although the interference signals representing specularly reflectedlight from the tear film are influenced by all layers in the tear film,the analysis of interference interactions due to the specularlyreflected light can be analyzed under a 2-wave tear film model (i.e.,two reflections) to measure LLT. A 2-wave tear film model is based on afirst light wave(s) specularly reflecting from the air-to-lipid layertransition of a tear film and a second light wave specularly reflectingfrom the lipid layer-to-aqueous layer transition of the tear film. Inthe 2-wave model, the aqueous layer is effective ignored and treated tobe of infinite thickness. To measure LLT using a 2-wave model, a 2-wavetear film model was developed wherein the light source and lipid layersof varying thicknesses were modeled mathematically. To model thetear-film interference portion, commercially available software, such asthat available by FilmStar and Zemax as examples, allows imagesimulation of thin films for modeling. Relevant effects that can beconsidered in the simulation include refraction, reflection, phasedifference, polarization, angle of incidence, and refractive indexwavelength dispersion. For example, a lipid layer could be modeled ashaving an index of refraction of 1.48, or as a BK7 fused silica havingan index of refraction of 1.52, or SiO₂ fused silica substrate having a1.46 index of refraction, as non-limiting examples. A back material,such as Magnesium Oxide (MgO) having an index of refraction of 1.74, orMagnesium Flouride (MgF₂) having an index of refraction of 1.38, may beused to provide a 2-wave model of air/(BK7 fused silica or SiO₂)/(BK7,MgO or MgF₂) (e.g., I/O/1.52/1.74 for air/BK7/MgO or 1.0/1.46/1.38 forair/SiO₂/MgF₂). To obtain the most accurate modeling results, the modelcan include the refractive index and wavelength dispersion values ofbiological lipid material and biological aqueous material, found fromthe literature, thus to provide a precise two-wave model ofair/lipid/aqueous layers. Thus, a 2-wave tear film interference modelallows measurement of LLT regardless of ALT.

Simulations can be mathematically performed by varying the LLT between10 to 300 nm. As a second step, the RGB color values of the resultinginterference signals from the modeled light source causing the modeledlipid layer to specularly reflected light and received by the modeledcamera were determined for each of the modeled LLT. These RGB colorvalues representing interference interactions in specularly reflectedlight from the modeled tear film were used to form a 2-wave model LLTpalette, wherein each RGB color value is assigned a different LLT. Theresulting subtracted image of the first and second images from thepatient's tear film containing interference signals representingspecularly reflected light are compared to the RGB color values in the2-wave model LLT palette to measure LLT.

In another embodiment, a 3-wave tear film interference model may beemployed to estimate LLT. A 3-wave tear film interference model does notassume that the aqueous layer is infinite in thickness. In an actualpatient's tear film, the aqueous layer is not infinite. The 3-wave tearfilm interference model is based on both the first and second reflectedlight waves of the 2-wave model and additionally light wave(s)specularly reflecting from the aqueous-to-mucin layer and/or corneatransitions. Thus, a 3-wave tear film interference model recognizes thecontribution of specularly reflected light from the aqueous-to-mucinlayer and/or cornea transition that the 2-wave tear film interferencemodel does not. To estimate LLT using a 3-wave tear film interferencemodel, a 3-wave tear film model was previously constructed wherein thelight source and a tear film of varying lipid and aqueous layerthicknesses were mathematically modeled. For example, a lipid layercould be mathematically modeled as a material having an index ofrefraction of 1.48 or as fused silica substrate (SiO₂), which has a 1.46index of refraction, or as BK7 fused silica having an index ofrefraction of 1.52. Different thicknesses of the lipid layer can besimulated. A fixed thickness aqueous layer (e.g., >=2 μm) could bemathematically modeled as Magnesium Flouride (MgF₂) having an index ofrefraction of 1.38 or Magnesium Oxide (MgO) having an index ofrefraction of 1.74. A biological cornea could be mathematically modeledas fused silica with no dispersion, thereby resulting in a 3-wave modelof air/SiO₂/MgF₂/SiO₂ (i.e., 1.0/1.46/1.38/1.46 with no dispersion) orair/BK7/MgO/BK7 (i.e., 1.0/1.52/1.74/1.52), as non-limiting examples. Asbefore, accurate results are obtained if the model can include therefractive index and wavelength dispersion values of biological lipidmaterial, biological aqueous material, and cornea tissue, found from theliterature, thus to provide a precise two-wave model ofair/lipid/aqueous/cornea layers. The resulting interference interactionsof specularly reflected light from the various LLT values and with afixed ALT value are recorded in the model and, when combined withmodeling of the light source and the camera, will be used to compareagainst interference from specularly reflected light from an actual tearfilm to measure LLT and/or ALT.

In another embodiment of the OSI device 170 and the post-processingsystem 362 in particular, a 3-wave tear film interference model isemployed to estimate both LLT and ALT. In this regard, instead ofproviding either a 2-wave theoretical tear film interference model thatassumes an infinite aqueous layer thickness or a 3-wave model thatassumes a fixed or minimum aqueous layer thickness (e.g., ≧2 μm), a3-wave theoretical tear film interference model is developed thatprovides variances in both LLT and ALT in the mathematical model of thetear film. Again, the lipid layer in the tear film model could bemodeled mathematically as a material having an index of refraction of1.48, as fused silica substrate (SiO₂) having a 1.46 index ofrefraction, or as BK7 fused silica having an index of refraction of 1.52as non-limiting examples. The aqueous layer could be modeledmathematically as Magnesium Flouride (MgF₂) having an index ofrefraction of 1.38 or Magnesium Oxide (MgO) having an index ofrefraction of 1.74. A biological cornea could be modeled as fused silicawith no dispersion, thereby resulting in a 3-wave model ofair/SiO₂/MgF₂/SiO₂ (no dispersion) or air/BK7/MgO/BK7, as non-limitingexamples. Once again, the most accurate results are obtained if themodel can include the refractive index and wavelength dispersion valuesof biological lipid material, biological aqueous material, and corneatissue, found from the literature, thus to provide a precise two-wavemodel of air/lipid/aqueous/cornea layers. Thus, a two-dimensional (2D)TFLT palette 560 (FIG. 37A) is produced for analysis of interferenceinteractions from specularly reflected light from the tear film. Onedimension of the TFLT palette 560 represents a range of RGB color valueseach representing a given theoretical LLT calculated by mathematicallymodeling the light source and the camera and calculating theinterference interactions from specularly reflected light from the tearfilm model for each variation in LLT 564 in the tear film interferencemodel. A second dimension of the TFLT palette 560 represents ALT alsocalculated by mathematically modeling the light source and the cameraand calculating the interference interactions from specularly reflectedlight from the tear film interference model for each variation in ALT562 at each LLT value 564 in the tear film interference model.

Post-Processing/TFLT Measurement

To measure TFLT, a spectral analysis of the resulting interferencesignal or image is performed during post-processing to calculate a TFLT.In one embodiment, the spectral analysis is performed by performing alook-up in a tear film interference model to compare one or moreinterference interactions present in the resulting interference signalrepresenting specularly reflected light from the tear film to the RGBcolor values in the tear film interference model. In this regard, FIGS.37A and 37B illustrate two examples of palette models for use inpost-processing of the resulting image having interference interactionsfrom specularly reflected light from the tear film using a 3-wavetheoretical tear film interference model developed using a 3-wavetheoretical tear film model. In general, an RGB numerical value colorscheme is employed in this embodiment, wherein the RGB value of a givenpixel from a resulting pre-processed tear film image of a patient iscompared to RGB values in the 3-wave tear film interference modelrepresenting color values for various LLTs and ALTs in a 3-wave modeledtheoretical tear film. The closest matching RGB color is used todetermine the LLT and/or ALT for each pixel in the resulting signal orimage. All pixels for a given resulting frame containing the resultinginterference signal are analyzed in the same manner on a pixel-by-pixelbasis. A histogram of the LLT and ALT occurrences is then developed forall pixels for all frames and the average LLT and ALT determined fromthe histogram (block 548 in FIG. 36).

FIG. 37A illustrates an exemplary TFLT palette 560 in the form of colorsrepresenting the included RGB color values representing interference ofspecularly reflected light from a 3-wave theoretical tear film modelused to compared colors from the resulting image of the patient's tearfilm to estimate LLT and ALT. FIG. 37B illustrates an alternativeexample of a TFLT palette 560′ in the form of colors representing theincluded RGB color values representing interference of specularlyreflected light from a 3-wave theoretical tear film model used tocompare colors from the resulting image of the patient's tear film toestimate LLT and ALT. As illustrated in FIG. 37A, the TFLT palette 560contains a plurality of hue colors arranged in a series of rows 562 andcolumns 564. In this example, there are 144 color hue entries in theTFLT palette 560, with nine (9) different ALTs and sixteen (16)different LLTs in the illustrated TFLT palette 560, although anotherembodiment includes thirty (30) different LLTs. Providing any number ofLLT and TFLT increments is theoretically possible. The columns 564 inthe TFLT palette 560 contain a series of LLTs in ascending order ofthickness from left to right. The rows 562 in the TFLT palette 560contain a series of ALTs in ascending order of thickness from top tobottom. The sixteen (16) LLT increments provided in the columns 564 inthe TFLT palette 560 are 25, 50, 75, 80, 90, 100, 113, 125, 138, 150,163, 175, 180, 190, 200, and 225 nanometers (nm). The nine (9) ALTincrements provided in the rows 562 in the TFLT palette 560 are 0.25,0.5, 0.75, 1.0, 1.25, 1.5, 1.75, 3.0 and 6.0 μm. As another example, asillustrated in FIG. 37B, the LLTs in the columns 564′ in the TFLTpalette 560′ are provided in increments of 10 nm between 0 nm and 160nm. The nine (9) ALT increments provided in the rows 562′ in the TFLTpalette 560 are 0.3, 0.5, 0.8, 1.0, 1.3, 1.5, 1.8, 2.0 and 5.0 μm.

As part of a per pixel LLT analysis 544 provided in the post-processingsystem 362 in FIG. 36, for each pixel in each of the pre-processedresulting images of the area or region of interest in the tear film, aclosest match determination is made between the RGB color of the pixelto the nearest RGB color in the TFLT palette 560 (block 545 in FIG. 36).The ALTs and LLTs for that pixel are determined by the corresponding ALTthickness in the y-axis of the TFLT palette 560, and the correspondingLLT thickness in the x-axis of the TFLT palette 560. As illustrated inFIG. 37, the TFLT palette 560 colors are actually represented by RGBvalues. The pixels in each of the pre-processed resulting images of thetear film are also converted and stored as RGB values, although anyother color representation can be used as desired, as long as thepalette and the image pixel data use the same representational colorspace. FIG. 38 illustrates the TFLT palette 560 in color pattern formwith normalization applied to each red-green-blue (RGB) color valueindividually. Normalizing a TFLT palette is optional. The TFLT palette560 in FIG. 38 is displayed using brightness control (i.e.,normalization, as previously described) and without the RGB valuesincluded, which may be more visually pleasing to a clinician ifdisplayed on the display 174. The GUI utility 380 allows selection ofdifferent palettes by selecting a file in the palette file drop down502, as illustrated in FIG. 27, each palette being specific to thechoice of 2-wave vs. 3-wave mode, the chosen source's spectrum, and thechosen camera's RGB spectral responses. To determine the closest pixelcolor in the TFLT palette 560, a Euclidean distance color differenceequation is employed to calculate the distance in color between the RGBvalue of a pixel from the pre-processed resulting image of the patient'stear film and RGB values in the TFLT palette 560 as follows below,although other embodiments of the present disclosure are not so limited:Diff.=√((Rpixel−Rpalette)²+(Gpixel−Gpalette)²+(Bpixel−Bpalette)²)

Thus, the color difference is calculated for all palette entries in theTFLT palette 560. The corresponding LLT and ALT values are determinedfrom the color hue in the TFLT palette 560 having the least differencefrom each pixel in each frame of the pre-processed resulting images ofthe tear film. The results can be stored in RAM(s) 358 or any otherconvenient storage medium. To prevent pixels without a close match to acolor in the TFLT palette 560 from being included in a processed resultof LLT and ALT, a setting can be made to discard pixels from the resultsif the distance between the color of a given pixel is not within theentered acceptable distance of a color value in the TFLT palette 560(block 546 in FIG. 36). The GUI utility 380 in FIG. 27 illustrates thissetting such as would be the case if made available to a technician orclinician. A distance range input box 508 is provided to allow themaximum distance value to be provided for a pixel in a tear film imageto be included in LLT and ALT results. Alternatively, all pixels can beincluded in the LLT and ALT results by selecting the ignore distanceselection box 510 in the GUI utility 380 of FIG. 27.

Each LLT and ALT determined for each pixel from a comparison in the TFLTpalette 560 via the closest matching color that is within a givendistance (if that post-processing setting 366 is set) or for all LLT andALT determined values are then used to build a TFLT histogram. The TFLThistogram is used to determine a weighted average of the LLT and ALTvalues for each pixel in the resulting image(s) of the patient's tearfilm to provide an overall estimate of the patient's LLT and ALT. FIG.39 illustrates an example of such a TFLT histogram 460. This TFLThistogram 570 may be displayed as a result of the shown LLT histogramselection box 500 being selected in the GUI utility 380 of FIG. 27. Asillustrated therein, for each pixel within an acceptable distance, theTFLT histogram 570 is built in a stacked fashion with determined ALTvalues 574 stacked for each determined LLT value 572 (block 549 in FIG.36). Thus, the TFLT histogram 570 represents LLT and ALT values for eachpixel. A horizontal line separates each stacked ALT value 574 withineach LLT bar.

One convenient way to determine the final LLT and ALT estimates is witha simple weighted average of the LLT and ALT values 572, 574 in the TFLThistogram 570. In the example of the TFLT histogram 570 in FIG. 39, theaverage LLT value 576 was determined to be 90.9 nm. The number ofsamples 578 (i.e., pixels) included in the TFLT histogram 570 was31,119. The frame number 580 indicates which frame of the resultingvideo image is being processed, since the TFLT histogram 570 representsa single frame result, or the first of a frame pair in the case ofbackground subtraction. The maximum distance 582 between the color ofany given pixel among the 31,119 pixels and a color in the TFLT palette560 was 19.9, 20 may have been the set limit (Maximum Acceptable PaletteDistance) for inclusion of any matches. The average distance 584 betweenthe color of each of the 31,119 pixels and its matching color in theTFLT palette 560 was 7.8. The maximum distance 582 and average distance584 values provide an indication of how well the color values of thepixels in the interference signal of the specularly reflected light fromthe patient's tear film match the color values in the TFLT palette 560.The smaller the distance, the closer the matches. The TFLT histogram 570can be displayed on the display 174 to allow a clinician to review thisinformation graphically as well as numerically. If either the maximumdistance 582 or average distance 584 values are too high, this may be anindication that the measured LLT and ALT values may be inaccurate, orthat the image normalization is not of the correct value. Furtherimaging of the patient's eye and tear film, or system recalibration canbe performed to attempt to improve the results. Also, a histogram 586 ofthe LLT distances 588 between the pixels and the colors in the TFLTpalette 560 can be displayed as illustrated in FIG. 40 to show thedistribution of the distance differences to further assist a clinicianin judgment of the results.

Other results can be displayed on the display 174 of the OSI device 170that may be used by a physician or technician to judge the LLT and/orALT measurement results. For example, FIG. 41 illustrates a thresholdwindow 590 illustrating a (inverse) threshold mask 592 that was usedduring pre-processing of the tear film images. In this example, thethreshold window 590 was generated as a result of the show thresholdwindow selection box 482 being selected in the GUI utility 380 of FIG.27. This may be used by a clinician to humanly evaluate whether thethreshold mask looks abnormal. If so, this may have caused the LLT andALT estimates to be inaccurate and may cause the clinician to discardthe results and image the patient's tear film again. The maximumdistance between the color of any given pixel among the 31,119 pixelsand a color in the TFLT palette 560 was 19.9 in this example.

FIG. 42 illustrates another histogram that may be displayed on thedisplay 174 and may be useful to a clinician. As illustrated therein, athree-dimensional (3D) histogram plot 460 is illustrated. The cliniciancan choose whether the OSI device 170 displays this histogram plot 460by selecting the 3D plot selection box 516 in the GUI utility 380 ofFIG. 27, as an example, or the OSI device 170 may automatically displaythe histogram plot 460. The 3D histogram plot 460 is simply another wayto graphically display the fit of the processed pixels from thepre-processed images of the tear film to the TFLT palette 560. The planedefined by the LLT 596 and ALT 598 axes represents the TFLT palette 560.The axis labeled “Samples” 600 is the number of pixels that match aparticular color in the TFLT palette 560.

FIG. 43 illustrates a result image 594 of the specularly reflected lightfrom a patient's tear film. However, the actual pixel value for a givenarea on the tear film is replaced with the determined closest matchingcolor value representation in the TFLT palette 560 to a given pixel forthat pixel location in the resulting image of the patient's tear film(block 547 in FIG. 36). This setting can be selected, for example, inthe GUI utility 380 of FIG. 27. Therein, a “replace resulting image . .. ” selection box 512 is provided to allow a clinician to choose thisoption. Visually displaying interference interactions representing theclosest matching color value to the interference interactions in theinterference signal of the specularly reflected light from a patient'stear film in this manner may be helpful to determine how closely thetear film interference model matches the actual color value representingthe resulting image (or pixels in the image).

Ambiguities can arise when calculating the nearest distance between anRGB value of a pixel from a tear film image and RGB values in a TFLTpalette, such as TFLT palettes 560 and 560′ in FIGS. 37A and 37B asexamples. This is because when the theoretical LLT of the TFLT paletteis plotted in RGB space for a given ALT in three-dimensional (3D) space,the TFLT palette 604 is a locus that resembles a pretzel like curve, asillustrated with a 2-D representation in the exemplary TFLT palettelocus 606 in FIG. 44. Ambiguities can arise when a tear film image RGBpixel value has close matches to the TFLT palette locus 606 atsignificantly different LLT levels. For example, as illustrated in theTFLT palette locus 606 in FIG. 44, there are three (3) areas of closeintersection 608, 610, 612 between RGB values in the TFLT palette locus606 even though these areas of close intersection 608, 610, 612represent substantially different LLTs on the TFLT palette locus 606.This is due to the cyclical phenomenon caused by increasing orders ofoptical wave interference, and in particular, first order versus secondorder interference for the LLT range in the tear films. Thus, if an RGBvalue of a tear film image pixel is sufficiently close to two differentLLT points in the TFLT palette locus 606, the closest RGB match may bedifficult to match. The closest RGB match may be to an incorrect LLT inthe TFLT palette locus 606 due to error in the camera and translation ofreceived light to RGB values. Thus, it may be desired to provide furtherprocessing when determining the closest RGB value in the TFLT palettelocus 606 to RGB values of tear film image pixel values when measuringTFLT.

In this regard, there are several possibilities that can be employed toavoid ambiguous RGB matches in a TFLT palette. For example, the maximumLLT values in a TFLT palette may be limited. For example, the TFLTpalette locus 606 in FIG. 44 includes LLTs between 10 nm and 300 nm. Ifthe TFLT palette locus 606 was limited in LLT range, such as 240 nm asillustrated in the TFLT palette locus 614 in FIG. 45, two areas of closeintersection 610 and 612 in the TFLT palette 604 in FIG. 44 are avoidedin the TFLT palette 604 of FIG. 45. This restriction of the LLT rangesmay be acceptable based on clinical experience since most patients donot exhibit tear film colors above the 240 nm range and dry eye symptomsare more problematic at thinner LLTs. In this scenario, the limited TFLTpalette 604 of FIG. 45 would be used as the TFLT palette in thepost-processing system 362 in FIG. 36, as an example.

Even by eliminating two areas of close intersection 610, 612 in the TFLTpalette 604, as illustrated in FIG. 45, the area of close intersection608 still remains in the TFLT palette locus 614. In this embodiment, thearea of close intersection 608 is for LLT values near 20 nm versus 180nm. In these regions, the maximum distance allowed for a valid RGB matchis restricted to a value of about half the distance of the TFLTpalette's 604 nearing ambiguity distance. In this regard, RGB values fortear film pixels with match distances exceeding the specified values canbe further excluded from the TFLT calculation to avoid tear film pixelshaving ambiguous corresponding LLT values for a given RGB value to avoiderror in TFLT measurement as a result.

In this regard, FIG. 46 illustrates the TFLT palette locus 614 in FIG.45, but with a circle of radius R swept along the path of the TFLTpalette locus 614 in a cylinder or pipe 616 of radius R. Radius R is theacceptable distance to palette (ADP), which can be configured in thecontrol system 340. When visualized as a swept volume inside thecylinder or pipe 616, RGB values of tear film image pixels that fallwithin those intersecting volumes may be considered ambiguous and thusnot used in calculating TFLT, including the average TFLT. The smallerthe ADP is set, the more poorly matching tear film image pixels that maybe excluded in TFLT measurement, but less pixels are available for usein calculation of TFLT. The larger the ADP is set, the less tear filmimage pixels that may be excluded in TFLT measurement, but it is morepossible that incorrect LLTs are included in the TFLT measurement. TheADP can be set to any value desired. Thus, the ADP acts effectively as afilter to filter out RGB values for tear film images that are deemed apoor match and those that may be ambiguous according to the ADP setting.This filtering can be included in the post-processing system 362 in FIG.36, as an example, and in step 546 therein, as an example.

Graphical User Interface (GUI)

In order to operate the OSI device 170, a user interface program may beprovided in the user interface system 378 (see FIG. 25A) that drivesvarious graphical user interface (GUI) screens on the display 174 of theOSI device 170 in addition to the GUI utility 380 of FIG. 27 to allowaccess to the OSI device 170. Some examples of control and accesses havebeen previously described above. Examples of these GUI screens from thisGUI are illustrated in FIGS. 44-48 and described below. The GUI screensallow access to the control system 340 in the OSI device 170 and tofeatures provided therein. As illustrated in FIG. 47, a login GUI screen620 is illustrated. The login GUI screen 620 may be provided in the formof a GUI window 621 that is initiated when a program is executed. Thelogin GUI screen 620 allows a clinician or other user to log into theOSI device 170. The OSI device 170 may have protected access such thatone must have an authorized user name and password to gain access. Thismay be provided to comply with medical records and privacy protectionlaws. As illustrated therein, a user can enter their user name in a username text box 622 and a corresponding password in the password text box624. A touch or virtual keyboard 626 may be provided to allowalphanumeric entry. To gain access to help or to log out, the user canselect the help and log out tabs 628, 630, which may remain resident andavailable on any of the GUI screens. After the user is ready to login,the user can select the submit button 632. The user name and passwordentered in the user name text box 622 and the password text box 624 areverified against permissible users in a user database stored in the diskmemory 368 in the OSI device 170 (see FIG. 25A).

If a user successfully logs into the OSI device 170, a patient GUIscreen 634 appears on the display 174 with the patient records tab 631selected, as illustrated in FIG. 48. The patient GUI screen 634 allows auser to either create a new patient or to access an existing patient. Anew patient or patient search information can be entered into any of thevarious patient text boxes 636 that correspond to patient fields in apatient database. Again, the information can be entered through thevirtual keyboard 626, facilitated with a mouse pointing device (notshown), a joystick, or with a touch screen covering on the display 174.These include a patient ID text box 638, patient last name text box 640,patient middle initial text box 642, a patient first name text box 644,and a date of birth text box 646. This data can be entered for a newpatient, or used to search a patient database on the disk memory 368(see FIG. 25A) to access an existing patient's records. The OSI device170 may contain disk memory 368 with enough storage capability to storeinformation and tear film images regarding a number of patients.Further, the OSI device 170 may be configured to store patientinformation outside of the OSI device 170 on a separate local memorystorage device or remotely. If the patient data added in the patienttext boxes 636 is for a new patient, the user can select the add newpatient button 652 to add the new patient to the patient database. Thepatients in the patient database can also be reviewed in a scroll box648. A scroll control 650 allows up and down scrolling of the patientdatabase records. The patient database records are shown as being sortedby last name, but may be sortable by any of the patient fields in thepatient database.

If a patient is selected in the scroll box 648, which may be an existingor just newly added patient, as illustrated in the GUI screen 660 inFIG. 49, the user is provided with an option to either capture new tearfilm images of the selected patient or to view past images, if past tearfilm images are stored for the selected patient on disk memory 368. Inthis regard, the selected patient is highlighted 662 in the patientscroll box 648, and a select patient action pop-up box 664 is displayed.The user can either select the capture new images button 666 or the viewpast images button 668. If the capture new images button 666 isselected, the capture images GUI 670 is displayed to the user under thecapture images tab 671 on the display 174, which is illustrated in FIG.50. As illustrated therein, a patient eye image viewing area 672 isprovided, which is providing images of the patient's eye and tear filmobtained by the video camera 198 in the OSI device 170. In this example,the image is of an overlay of the subtracted first and second patternimages of the patient's tear film onto the raw image of the patient'seye and tear film, as previously discussed. The focus of the image canbe adjusted via a focus control 674. The brightness level of the imagein the viewing area 672 is controlled via a brightness control 676. Theuser can control the position of the video camera 198 to align thecamera lens with the tear film of interest whether the lens is alignedwith the patient's left or right eye via an eye selection control 678.Each frame of the patient's eye captured by the video camera 198 can bestepped via a stepping control 680. Optionally, or in addition, ajoystick may be provided in the OSI device 170 to allow control of thevideo camera 198.

The stored images of the patient's eye and tear film can also beaccessed from a patient history database stored in disk memory 368. FIG.51 illustrates a patient history GUI screen 682 that shows a pop-upwindow 684 showing historical entries for a given patient. For each tearfilm imaging, a time and date stamp 685 is provided. The images of apatient's left and right eye can be shown in thumbnail views 686, 688for ease in selection by a user. The stored images can be scrolled upand down in the pop-up window 684 via a step scroll bar 690. Label namesin tag boxes 692 can also be associated with the images. Once a desiredimage is selected for display, the user can select the image to displaythe image in larger view in the capture images GUI 670 in FIG. 50.Further, two tear film images of a patient can be simultaneouslydisplayed from any current or prior examinations for a single patient,as illustrated in FIG. 52.

As illustrated in FIG. 52, a view images GUI screen 700 is shown,wherein a user has selected a view images tab 701 to display images of apatient's ocular tear film. In this view images GUI screen 700, bothimages of the patient's left eye 702 and right eye 704 are illustratedside by side. In this example, the images 702, 704 are overlays of thesubtracted first and second pattern images of the patients tear filmonto the raw image of the patient's tear eye and tear film, aspreviously discussed. Scroll buttons 706, 708 can be selected to move adesired image among the video of images of the patient's eye for displayin the view images GUI screen 700. Further, step and play controls 710,712 allow the user to control playing a stored video of the patient'stear film images and stepping through the patient's tear film images oneat a time, if desired. The user can also select an open patient historytab 714 to review information stored regarding the patient's history,which may aid in analysis and determining whether the patient's tearfilm has improved or degraded. A toggle button 715 can be selected bythe user to switch the images 702, 704 from the overlay view to just theimages 720, 722, of the resulting interference interactions ofspecularly reflected light from the patient's tear films, as illustratedin FIG. 53. As illustrated in FIG. 53, only the resulting interferenceinteractions from the patient's tear film are illustrated. The user mayselect this option if it is desired to concentrate the visualexamination of the patient's tear film solely to the interferenceinteractions.

In yet another embodiment, a polarization subtraction algorithm improvesisolation of a tear film interference pattern from a background signal.FIG. 54 is a flow chart that illustrates step-by-step computerizedprocessing of the polarization subtraction algorithm of the presentdisclosure. A first image frame shown in FIG. 55A is automaticallyextracted from a polarization video segment (block 900). A second imageframe shown in FIG. 55B is automatically extracted from a perpendicularpolarization video segment (block 902). The polarization subtractionalgorithm automatically then selects a plurality of points (e.g.,pixels) within the first image frame (block 904). In the exemplary caseof FIG. 55A, four selected points are highlighted in green. Next, thepolarization subtraction algorithm automatically selects homologouspoints. The homologous points are highlighted in green in the secondimage frame shown in FIG. 55B.

Once the homologous points are selected, an imaging processing toolinvoked by the polarization subtraction algorithm uses pairs of theplurality of points and pairs of the homologous points to calculate aspatial transformation (block 904). For example, this processing stepcould be performed using the Matlab® function “cp2tform.” After thespatial transformation has been calculated, the polarization subtractionalgorithm transforms the second image frame by invoking an imagingprocessing tool that transforms the second image in accordance with thepreviously calculated spatial transformation (block 910). In this case,the process of block (910) could be performed using the Matlab® function“imtransform.” FIG. 55C illustrates the resulting transformation of thesecond image frame.

Next, the polarization subtraction algorithm automatically selectshomologous regions of a sclera captured in the transformed first imageframe shown in FIG. 55C and the first image frame shown in FIG. 55D(block 912). Once the homologous regions are selected, the polarizationsubtraction algorithm automatically calculates a ratio of meanintensities in the selected homologous regions between the first imageframe and the second image frame is calculated (block 914).

Next, based on the calculated intensity ratio, the intensities in thesecond image frame are scaled automatically by the polarizationsubtraction algorithm (block 916). FIG. 55E shows an example of aresulting transformed and intensity scaled second image frame. At thispoint, the polarizer subtraction algorithm subtracts the transformed andintensity scaled second image frame from the first image frame, whichresults in a polarization subtraction image such as the polarizationsubtraction image shown in FIG. 55FF.

Many modifications and other embodiments of the present disclosure setforth herein will come to mind to one skilled in the art to which thedisclosure pertains having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. These modificationsinclude, but are not limited to, the type of light source orilluminator, the number of polarizers, the type of imaging device, imagedevice settings, the relationship between the illuminator and an imagingdevice, the control system, the type of tear film interference model,and the type of electronics or software employed therein, the display,the data storage associated with the OSI device for storing information,which may also be stored separately in a local or remotely locatedremote server or database from the OSI device, any input or outputdevices, settings, including pre-processing and post-processingsettings. Note that subtracting the second image from the first image asdisclosed herein includes combining the first and second images, whereinlike signals present in the first and second images are cancelled whencombined. Further, the present disclosure is not limited to illuminationof any particular area on the patient's tear film or use of anyparticular color value representation scheme.

Therefore, it is to be understood that the present disclosure is not tobe limited to the specific embodiments disclosed and that modificationsand other embodiments are intended to be included within the scope ofthe appended claims. It is intended that the present disclosure coverthe modifications and variations of this disclosure provided they comewithin the scope of the appended claims and their equivalents. Althoughspecific terms are employed herein, they are used in a generic anddescriptive sense only and not for purposes of limitation.

What is claimed is:
 1. An apparatus for imaging an ocular tear film,comprising: a control system configured to: (a) receive at least onefirst image containing optical wave interference of specularly reflectedlight in a first polarization plane along with a background signal froma region of interest (ROI) of an ocular tear film captured by an imagingdevice while illuminated by a multi-wavelength light source; (b) receiveat least one second image containing only the background signal in asecond polarization plane perpendicular or substantially perpendicularto the first polarization plane from the ROI of the ocular tear filmcaptured by an imaging device; (c) subtract the at least one secondimage from the at least one first image to generate at least oneresulting image containing the optical wave interference of specularlyreflected light from the ROI of the ocular tear film with the backgroundsignal removed or reduced.
 2. The apparatus of claim 1, furthercomprising an imaging device configured to capture the at least onefirst image and capture the at least one second image.
 3. The apparatusof claim 2, further comprising a polarizer disposed in an imaging pathof the imaging device, wherein the polarizer has a center axis and isselectively rotatable about the center axis to provide a firstpolarization axis relative to the first polarization plane of thespecularly reflected light from the ROI of the ocular tear film during acapture of the at least one first image, and wherein the polarizer isselectively rotatable to provide a second polarization axis that isperpendicular or substantially perpendicular relative to the firstpolarization axis during the capture of the at least one second image.4. The apparatus of claim 2, further comprising a rotatable wheel havinga first polarizer and second polarizer, the rotatable wheel selectivelyrotatable such that the first polarizer is disposed in an imaging pathof the imaging device during the capture of the at least one firstimage, and wherein the second polarizer is alternately disposed in theimaging path of the imaging device during the capture of the at leastone second image.
 5. The apparatus of claim 4, wherein the firstpolarizer has a first polarization axis that is parallel orsubstantially parallel to the first polarization plane of the specularlyreflected light from the ROI of the ocular tear film to pass orsubstantially pass the specularly reflected light to the imaging devicewhile reducing the background signal.
 6. The apparatus of claim 4,wherein the second polarizer has a second polarization axis that isperpendicular or substantially perpendicular to the second polarizationplane of the specularly reflected light from the ROI of the ocular tearfilm to reduce or eliminate the specularly reflected light while passinga portion of the background signal to the imaging device.
 7. Theapparatus of claim 4, wherein the first polarizer has a firstpolarization axis that is parallel or substantially parallel to thepolarization plane of the specularly reflected light from the ROI of theocular tear film to pass or substantially pass the specularly reflectedlight to the imaging device while reducing the background signal, andwherein the second polarizer has a second polarization axis that isperpendicular or substantially perpendicular to the polarization planeof the specularly reflected light from the ROI of the ocular tear filmto eliminate or substantially eliminate the specularly reflected lightwhile passing a portion of background signal to the imaging device. 8.The apparatus of claim 2, further comprising: a first polarizerselectively disposable in an imaging path of the imaging device during acapture of the at least one first image; and a second polarizerselectively disposable in the imaging path of the imaging device duringthe capture of the at least one second image.
 9. The apparatus of claim8, wherein the first polarizer has a first polarization axis that isparallel or substantially parallel to the first polarization plane ofthe specularly reflected light from the ROI of the ocular tear film topass or substantially pass the specularly reflected light to the imagingdevice while reducing the background signal before the background signalreaches the imaging device.
 10. The apparatus of claim 8, wherein thesecond polarizer has a second polarization axis that is perpendicular orsubstantially perpendicular to the second polarization plane of thespecularly reflected light from the ROI of the ocular tear film toreduce or eliminate the specularly reflected light before the specularlyreflected light reaches the imaging device while passing a portion ofthe background signal.
 11. The apparatus of claim 8, further comprising:a first polarizer selectively disposed in the imaging path of theimaging device, wherein the first polarizer has a polarization axis thatis parallel or substantially parallel to the first polarization plane ofthe specularly reflected light from the ROI of the ocular tear film topass or substantially pass the specularly reflected light to the imagingdevice while reducing the background signal before the background signalreaches the imaging device; and a second polarizer alternately disposedin the imaging path of the imaging device, wherein the second polarizerhas a polarization axis that is perpendicular or substantiallyperpendicular to the second polarization plane of the specularlyreflected light from the ROI of the ocular tear film to reduce oreliminate the specularly reflected light while passing a portion ofbackground signal to the imaging device.
 12. The apparatus of claim 8,wherein the first polarizer and the second polarizer are alternatelytranslatable into the imaging path of the imaging device.
 13. Theapparatus of claim 8, wherein the first polarizer and the secondpolarizer are alternately rotatable into the imaging path of the imagingdevice.
 14. The apparatus of claim 1, further comprising a first imagingdevice configured to capture the at least one first image, and a secondimaging device configured to capture the at least one second image. 15.The apparatus of claim 14, further comprising a non-polarizing beamsplitter configured to direct a first portion of the specularlyreflected light including the background signal to the first imagingdevice while simultaneously directing a second portion of the specularlyreflected light including the background signal to the second imagingdevice.
 16. The apparatus of claim 15, further comprising a firstpolarizer disposed in an imaging path of the first imaging device duringa capture of the at least one first image.
 17. The apparatus of claim16, wherein the first polarizer has a first polarization axis parallelor substantially parallel to the first polarization plane of thespecularly reflected light directed to the imaging device from the ROIof the ocular tear film to pass or substantially pass the specularlyreflected light to the first imaging device while reducing thebackground signal before the background signal reaches the first imagingdevice.
 18. The apparatus of claim 15, further comprising a secondpolarizer disposed in an imaging path of the second imaging deviceduring a capture of the at least one second image.
 19. The apparatus ofclaim 18, wherein the second polarizer has a second polarization axisperpendicular or substantially perpendicular to the second polarizationplane of the specularly reflected light from the ROI of the ocular tearfilm to reduce or eliminate the specularly reflected light before thespecularly reflected light reaches the second imaging device whilepassing a portion of the background signal to the second imaging device.20. The apparatus of claim 15, further comprising: a first polarizerdisposed in an imaging path of the first imaging device, wherein thefirst polarizer has a first polarization axis that is parallel orsubstantially parallel to the first polarization plane of the specularlyreflected light from the ROI of the ocular tear film to pass orsubstantially pass the specularly reflected light to the first imagingdevice while reducing the background signal; and a second polarizerdisposed in the imaging path of the second imaging device, wherein thesecond polarizer has a second polarization axis that is perpendicular orsubstantially perpendicular to the polarization plane of the specularlyreflected light from the ROI of the ocular tear film to reduce oreliminate the specularly reflected light while passing a portion ofbackground signal to the second imaging device.
 21. The apparatus ofclaim 14, further comprising a polarizing beam splitter configured todirect a portion of the specularly reflected light and the backgroundsignal to the first imaging device while simultaneously directing thebackground signal to the second imaging device reducing or eliminatingthe specularly reflected light.
 22. The apparatus of claim 1, furthercomprising a polarizer disposed in an illumination path of themulti-wavelength light source and configured to polarize light emittedfrom the multi-wavelength light source illuminating the ROI of theocular tear film.
 23. The apparatus of claim 22, wherein the polarizeris a circular polarizer.
 24. The apparatus of claim 1, wherein thebackground signal includes at least one of stray light and ambientlight.
 25. The apparatus of claim 1, wherein the imaging device isconfigured to capture the at least one second image when themulti-wavelength light source is illuminating the ROI of the ocular tearfilm.
 26. The apparatus of claim 25, wherein the background signalincludes diffusely reflected light resulting from illumination of theocular tear film.
 27. The apparatus of claim 26, wherein the backgroundsignal includes diffusely reflected light from an iris of an eye. 28.The apparatus of claim 1, wherein the control system is furtherconfigured to sequentially control an imaging device to capture theoptical wave interference of specularly reflected light and thebackground signal in the at least one first image to provide a pluralityof first images, and capture the background signal in the at least onesecond image to provide a plurality of second images interleaved withthe plurality of first images to provide a plurality of first and secondimage pairs.
 29. The apparatus of claim 28, wherein the control systemis configured to subtract the at least one second images from theplurality of first and second image pairs from the corresponding atleast one first images from the plurality of first and second imagepairs to generate a plurality of resulting images each containing theoptical wave interference of specularly reflected light from the ROI ofthe ocular tear film with the background signal removed or reduced. 30.The apparatus of claim 1, wherein the multi-wavelength light source iscomprised of a multi-wavelength Lambertian light source configured touniformly or substantially uniformly illuminate the ROI of the oculartear film.
 31. The apparatus of claim 1, wherein the control system isfurther configured to display at least one of the at least one firstimage, the at least one second image, or the at least one resultingimage on a visual display.
 32. A method of imaging an ocular tear film,comprising: illuminating a region of interest (ROI) of an ocular tearfilm with a multi-wavelength light source; capturing optical waveinterference of specularly reflected light in a first polarization planeincluding a background signal from the ROI of the ocular tear film whileilluminated by the multi-wavelength light source in at least one firstimage by an imaging device; capturing only the background signal in asecond polarizing plane perpendicular or substantially perpendicular tothe first polarization plane from the ROI of the ocular tear film in atleast one second image by an imaging device; and subtracting the atleast one second image from the at least one first image to generate atleast one resulting image containing the optical wave interference ofspecularly reflected light from the ROI of the ocular tear film with thebackground signal removed or reduced.